I can get behind that, based on his behavior in Zero, but it's not as though Waver isn't a sentimental person. Case Files also goes out of it's way to contrast Waver's obsession and love for Iskandar with the obsessions of other characters - the anime, in particular, makes a big point of the fact that living for the past is dangerous. Waver recognizing that in some small way by admitting he might've been happier if he had paid attention to Camus, but acknowledging that he hadn't and that's the choice they have to live with, felt like a huge moment of character growth for him.. and that might seem foreign, I guess, given that in all of his other appearances, Waver takes every opportunity he can to nostalgiatrip about Iskandar? But Case Files is The Waver Series, and seems overall to have a focus on accepting and moving on from grief, so I was pretty pleased with the general themes of the special.
Sorry, I know that was pretty tangential. Tl;dr I didn't think it was OOC because it just seemed like genuine character growth to me, but I recognize that others might not agree.