Where's it say that Alaric believes them to be traitors though? It seems to be the general narrative that the Dragoons were traitors. Where did that narrative it come?
From 100 years of cultural development of which we are not privy to the details? It's hard to say precisely when the nitty-gritty is in the details. To write a realistic depiction, I would say that no one Clanner stood up and declared that the Dragoons were actually traitors. It came from minuscule changes in attitudes, writings, and culture. Perhaps, over time, the strongly Crusader Wolves developed a pure rejection of the pre-Revival Warden Wolves, including the actions of Khans like Kerlin Ward and Ulric Kerensky. They ignore the purely factual, legal elements of the history and replace it with their own emotional reactions, inspired by Vlad Ward's ability to re-make themselves.
I'm merely positing that a century of changing perspectives and biases can create ideas that seem illogical to those who have a dispassionate view derived from nearly fact-perfect sourcebooks. Vikings did not have horns on their helmets, but that misconception remains today - amongst thousands of historical falsehoods. As another example, take a man thought by some to be a traitor. Was Napoleon the Usurper or the Emperor? Ask a Frenchman of 1805, of 1815, of 1840, of 1870, or of 1940. The answers will vary by person, by year, and by the cultural changes that define a nation's identity. You could tell a 3151 Clanner about the facts of the Dragoons case and they will have their own passionate ideas about what truly matters. Perhaps they believe that if the Dragoons stayed true to the Crusader cause, the Clans would have won on Tukayyid. That matters more than the Dragoons' orders.
The Clans have been in contact with the Dragoons for 100 years. If they were traitors, why did Wolf In Exile come to their aid on Outreach? The whole narrative feels forced. The Dragoons being Mercenaries, no question. Traitors; no.
Most likely, the issue is complicated. There are a multitude of perspectives. Maybe it's a Crusader v. Warden thing. Maybe the Exiles were more strongly influenced by Natasha Kerensky's return. Maybe the Exiles were sympathetic because of the similarities between the two groups as exiled Clanners in the Inner Sphere. Clanners are not a monolithic body of people.