Then they didn’t know/understand the history of Baptists.
It’s a problem now that churches don’t educate members in the basics of denominations. (Not that a particular denomination is the only “right” one.) The fundamental tenets and history were taught in our local church in youth group.
I understand that some Baptists consider themselves Protestant to differentiate from Catholic, Jewish, Muslim, etc. In that respect, it’s true. Technically, though, it isn’t correct.