He's not a complete idiot to entirely discard someone's upbringing in context of how someone turns out. But that's the tiny "you're a credit to your people" moment deviating from his usual approach.
And as for the other thing - I doubt it was long at all, seeing as he didn't even try and approach city elves (that much can be confirmed). But you're right, it's not stated how quickly he has given up on them. All we can say is that subtlety he exhibits now is generally comparable to sledgehammer.
He's not exactly subtle in criticizing anything he thinks is deserving of it - which is, like, everything 
Elves, dwarves, humans, Chantry, etc.... even Elvenhan (in banter) which he called no better than Tevinter at its time - he also chastised Dorian for romanticizing elves of old.
For someone who is considered a trickster and deceiver, he's astoundingly honest in his opinions, lol. Hence him not exactly coddling the Dalish is not a surprise - it's just that type of character: probably way too bitter, with hopes broken way too many times to not be pessimistic about... well, if not everything, then most things.
First of all, Dalish don't even know for a fact that Vallaslin actually dates back to Arlathan, much less nuances of its use then. And Elvhenan being destroyed by humans (with no pesky details like civil war that made it possible) is the version perpetuated by Tevinter - the truth has been lost long before Dales came to existence.
A good reason to be bitter about, especially if one approaches the clans and tries to teach the truth, only to be dismissed as a lunatic or worse.
As for vallaslin, note that the "bullshit" part does not, in fact (and despite what dialog wheel paraphrase would suggest) express disbelief as much as anger - he refuses to teach the Dalish but suddenly he approaches her with this information of what the marks "really" mean
Well, at least according to him, she's actually a person who *can* listen and deserves to know, even if she's initially angry about it. I mean, let's face it - for all the imagined personalities individual Inquisitors have in players' heads, to befriend - and especially romance - Solas, one has to assume that given Inquisitor is open-minded enough to accept harsh truths... otherwise I can't see the friendship/romance working at all: the bickering and frustration would be endless xD
Also, it's not that he denied that they honored the gods - but their main purpose was to mark slaves.
Though it should be said that the whole vallaslin thing was something he came up with in a spur of a moment (which still has to mean he was severely bothered by it) - as he was about to reveal the truth about himself (and that's according to his writer).
Though frankly (I've said it a couple times but I can repeat once more
) I find it hard to consider Vallaslin origin so shocking that it would turn Dalish culture upside down... Basically, there were masters that chose what god their slave would honor with markings on his face. Now Dalish choose their markings as the first adult decision in their life. I can see how one evolved into the other, really - isn't freedom all about being your own master, with control over your own life?
Um, how is being one's own master and controlling one's life symbolized by a.) giving in to what tradition dictates, with no questioning of how this tradition began in the first place, and b.) obtaining a lifelong mark to honor and follow a given god?
It's like saying that marriage was the first adult decision in a woman's life, back when it was expected for her to only marry and have kids, AND with a husband her family picks...
In fact, I think this is exactly what bothers Solas so much about vallaslin - it's sort of a double-slavery thing, for those that were enslaved; they were forced to not just serve the higher caste, but also forced to follow gods that their masters have followed.
Not to mention that it's probably a double insult for the Dread Wolf himself, seeing how passionate he is about freedom - to see people he probably freed from a hopeless fate embracing the symbol of what he rebelled against. The irony of that is just too grand to be dismissed as a mere change in culture (due to lack of knowledge or else). Dunno about you, but I'd harbor some resentment about it.
Either way, with all the things they got wrong, Dalish appear to be remarkably right about certain other things: their gods existed, immortality was apparently a thing, there IS powerful elven magic still left in the world and it's still beyond human ability to replicate it. They are also actively seeking out old lore and while we don't get many examples of them actually getting their hands on something important, we hear that apparently they do their best to exchange what they learn among clans and the one unambiguous example when presented with solid evidence of something uncomfortable, they behave MUCH better than humans.
The fact that they behave better than humans is a no-brainer, seeing how humans are portrayed as more bellicose than other races, at least in modern Thedas. But I'm not sure it could be claimed that it's been only through the Dalish that the knowledge of their gods, immortality of elves or powerful magic existed - I don't think even the Chantry denied the existence of elven gods, even if they claim that they're false gods (just like do with Old Gods of Tevinter - though we don't really know what either were).
As for exchanging knowledge... well no, for me this isn't apparent at all, seeing how scattered clans are and how different is their approach to, say, the outside world (with some Dalish clans being little else than bandits, some disappearing in forests entirely and some openly trading with humans, etc).