As a new FT8 user, I've only used WSJT-X software so far. I gather from what I've read online that some of the variants do provide more filtering capability.

Do I infer correctly that the WSJT team views their task as providing core functionality and that, by providing open-source software, they invite others to supply the bells and whistles?

This seems to me like a perfectly reasonable strategy. So those of us who want more functionality can explore other options and might very well find what were looking for in another package. If we like different things about different packages, then if those things happen to correlate with one operating objective or another (e.g., if one package has better filtering that's useful during unusual DX openings), then we can choose our software for a given operating session in light of our objectives for that session.