He fanboys over the romans in general. Seriously, anything bad against the romans gets him in a tizzy, and even though that's true, Arminius' damage was already done.
"damage" though
A bunch of Romans died, then a bunch of people who were not Romans died. That was his damage. Life went on, unaffected by Arminius' little war. Romans were incalculably more prosperous and powerful than the people on the other side of their frontiers. They regularly launched punitive expeditions over the Rhine and Danube that could not be countered. They enslaved countless people from over the frontier, they ensured that all tribes were led by their handpicked chiefs, they ensured that the rudimentary economies of those tribes were based entirely on Rome, and they took the pick of those tribes' fighting men for their own armies.
Arminius' victory in the Varian disaster mostly meant that those chieftains that Rome put into power wouldn't get to call themselves Roman citizens like the chieftains in Gaul and Britain did. Basically all the other modes of Roman control were still in place, and remained in place.
When Rome finally died in the West, it was because Romans killed it. The so-called "barbarians" were barely involved. And the kingdoms that eventually grew up in what used to be the Roman Empire bore no resemblance to the "barbarians" that allegedly birthed them, for the excellent reason that
those "barbarians" didn't birth them in the first place. And even the "barbarians" that were involved with the fall of Rome had very little to do with each other, and essentially nothing to do with the people who had lived there in Arminius' time.
Arminius did not inflict a world-historical defeat on the Roman superpower. He did not presage Alareiks, Attila, or Odovacar. He was another tinpot warlord who fell victim to Rome's infinite men and infinite money like all the rest, except it took him longer to do so.
Rome sucked, but it sucked because of outrageous classism, dictatorship, patriarchy, slavery, unimaginable brutality in war, and the usual train of premodern diseases and famines that made life for average individuals so freaking awful. Other stuff is irrelevant, wrong, or entirely made-up.