True generally, but for someone who's been in an OD-A for 20 years?
Green Berets aren't a specialised unit, they're the most versatile of special operations units. With that much time in and the amount of cross-training he'll have had at least basic knowledge and competence across the board. Being a jack-of-all-trades is kind of their thing.
And I've never ever heard any S(O)F member say anything positive about a Desert Eagle for actual use. It's like a Mk.23, but even far, far worse.
Maybe he was a nutjob, but how ingrained such training and experience is I find it hard to believe that he wouldn't have fallen back on it.
None of it makes sense to me.