Annnnnd out of likes. Drat.
I'm guessing they're using "traitor" in the sense that he's hiding this big secret from you that gets revealed later on, if he did cause it. He's joining you on false pretenses.
I... would call that a lie by omission, really. If he's joining to help fix the Breach, it's not necessarily false pretenses... just missing the little detail that he, whoops, caused it.
I think it's the former. Which was why I kind of objected before to the use of "traitor". It's fair to say that Anders betrayed Hawke. I don't think it's fair to accuse Solas of betraying the Inquisitor. Betraying the world perhaps? But I think that's iffy if he really didn't know what the outcome would be.
Though it'd be fair for the Inquisitor to be pissed at him if you do his romance and he never reveals himself as the person who opened the breach until he's forced to. But then again, I can see it from his perspective as well; he's probably feeling really guilty and afraid that the Inquisitor will turn away from him if he reveals himself.
Eh, I can see this from his angle pretty well. If I f**ked up that badly, no way would I want to admit that to anyone. Especially not someone I was starting to feel romantic for. I can tooootally see that. Poor guy. 
I've ran out of likes and I totally agree with you DragonRacer. If he didn't mean to do what he did and joins to help fix the problem..that's not exactly a traitor.
GDL did call him "nails".
The fact that he said "moral", too, helps ease some tension.
This is starting to make me feel better from yesterday, if it's shaping out to be like this, honestly. My woobie/maternal instincts are starting to kick in. I'm a gigantic sap for redemption journies. Lord. I'm having flashbacks. I was part of the Loghain Defense Brigade five years ago. I'm... getting a feeling I'll be Solas Defense Squad now, too. LMAO