This is interesting. I've always been told that a flutter cannot tolerate boiling water. And, since you blow into it instead of from it, I would think that handwashing with soap and hot water would clean it enough that it would be fine. I mean, we breathe from a spacer and those are to only be handwashed also.
Anyway, I'm glad it did not break on you, and please let me kow if you continue to sterilize it with no problems! The cleaner the better!