There's no historical account making this claim. Not in the novels, not in World of Thedas, not even in the games themselves. In fact, the timeline from WoT cites the "quickening" and the elven withdrawal from humanity as one of the earliest records on the timeline in -2850.
The elves of the Dales refused to aid Orlais. That's what the human historical account reads. Orlais - the nation that was created when Drakon lead a series of Exalted Marches against his neighbors, and imposed his particular Cult of the Maker as the national religion. There's also Iloren and his elven brethren fighting darspawn in the Anderfels during the Second Blight. And given how they were dealing with pressures from Orlais since Drakon's reign, if the elven historical account is correct, then the Dales retaliated against an imperialistic empire that was threatening everything Shartan and his elven rebellion fought so hard for.
Being nomadic is a matter of survival - elven culture is outlawed and the elven religion is criminalized, and outsiders tend to run them off, threaten them to convert, or attack them. The Dalish are criminals simply by refusing to surrender their way of life, as well as their adamant refusal to convert to the human religion. Pointing out that the Dalish are wary of outsiders makes sense when you consider the facts.
The city elves who live in dilapidated ghettos, are viewed as less than people by humans, and risk being purged en mass when they protest their living conditions or the injustices imposed on them. The last purge in Denerim even lead to the massacre of the Denerim Alienage orphanage. Even Halamshiral rebelled due to how the elves are treated by humans.
Considering one of the racial options is for a Dalish elf, perhaps the Inquisition can help the Dalish.
The Arlathan elves were one of two advanced(and seemingly very magically advanced) native societies when the humans arrived. The accounts are very clear in that the humans don't arrive in anything approaching a unified invasive force(like the Qunari), and spend centuries developing their societies. The quickening is irrelevant to the elves ignoring the development of human nations and society. The elves had met the dwarves 1500 years prior to the arrival of the humans, unless they were hostile to them, there should be no reason that the elves could not keep track of human society post Iron Wall phase through dwarves.
The codexes indicate that the elves believed themselves to be immortal as well as the dominant race in Thedas. Now I suppose we could speculate that the Arlathan elves were actually weaker than the myriad individual human tribes and local kingdoms, but that would mean they warrant much less attention than the Dalish people give them, as the Dales would've been a far more powerful country than them.
Deciding not to intervene with the rise of Tevinter is a risk the elves chose to take(much like every other human tribe/kingdom at the time took). As the dominant nation in Thedas at the time, they let someone else(a very fanatical and blood crazy someone else) become the new dominant nation. The foolish isolationist policy of elven leaders led to downfall of Elvenhan. Cynically speaking, the least they should've done was pop out and wipe out any threat that they heard of every couple centuries or so. Even if their nation were human, these same isolationist policies would've led to their downfall(the mistakes are independent of their race)
The elves didn't help anyone during the second Blight. The second Blight encompassed the whole world. As a result, they had no friends after the second Blight. Orlais had many rivals and enemies the elves could've endeavored to befriend. This stance is idiotic regardless of whether they were a human or elven nation(and would've resulted in them being conquered even if they were human).
For the most part, racism is a comprehensible part of human history. Doesn't make it rational or effective. A lot of human behavior is that way(and lets face it, Thedas elven psychology is closely modeled after human psychology), like the tendency towards immediate gratification. At the minimum, elven leaders should try and maintain friendly relations with all prominent city leaders along their nomadic route(nothing has ever indicated that they even maintain communications with these people) so as to minimize conflict. Even hostile nations have some dialogue.
Well not all city elves live in ghettos, not all city elves are poor(and for the most part, Dalish people are probably even poorer in terms of currency and mercantile power), and not all city elves lack influence. As it stands, the City elves have a much greater chance of gaining equal status through either military or economic means. Human attitudes towards city elves are more benign than towards Dalish, the City Elves are actually numerous(Alienages existing in numerous locations),and we have met at least one wealthy elf before.
The greatest problem with Inquisition simply helping the Dalish, is they'd probably lose the country again(sans talented Dalish leadership). Even assuming that isolationism was functional, from a purely military standpoint, its necessary to understand foreign cultures and their ways of thinking.
A city elf is far more likely to be wordly than a Dalish elf(by virtue of dramatically more exposure to a variety of cultures and ideas), which actually makes them the best source for educated leadership amongst the elves imo.
To me, the City Elves appear to be doing everything within their power to advance(and are being unfairly treated as a result of the actions of their ancestors, but such is life in Thedas; least they weren't born a Tevinter slave), while the Dalish are still shirking the bare minimum to living alongside other peoples.