Well, I'm not sure why I put "stupidly" rather than my more usual "foolishly", but you're absolutely right to pick me up on it. I think there are two reasons, both of them bad!
1) It's self-deprecating. "Why did I take the trouble to email them in my lame Welsh when it's probably awful?" Bad because it's (a) almost certainly not true, and in any case (b) if a thing's worth doing, it's worth doing badly. (If you wait until your Welsh is 'perfect' before using it with people, you'll never use it.)
2) I could have communicated with them much more efficiently in English, because they're bound to be bilingual and I'm not. But this goes to the heart of "What is the point of using a minority language?" I never really have to confront this when people exclaim over my using Catalan rather than Spanish, because I honestly don't speak terribly good Spanish -- if I try, it'll end up turning back into Catalan anyway. So although I get brownie points from Catalan speakers, I'm actually just communicating with them as best I can. But that's never going to be the case with my Welsh, outside of Patagonia -- and I have to recognise (like any learner) that it's still worth making the effort to do it anyway.
So, yes -- I was probably right to email them in Welsh, and you were certainly right to pick me up on it.