At base your observation about the Donatists is a good one. Fundamentally, why would Baptists care if they are Donatists? Whether Baptists are viewed as a new form of heretic or a reinvigorated old form by The Church doubtless makes no difference to them...which is why they shouldn't pretend they are The Church, rather accept that they are something else entirely which is good enough for them (ecclesial assembly?).
On this topic, from me, base is as good as it's going to get, but thanks for the compliment. :) And because I don't know the ins and outs, I didn't want to associate or disassociate from something about which I know little.
You said that we shouldn't pretend to be part of the Church, do you think we do? As far as I know we do believe that we are something else outside the RC Church, and we are comfortable with that.