So Stan, are you saying you are unable to separate the theories of a person who makes a recording from the recording itself? That's too bad. That will hamper your ability to make progress as a scientist, and in life in general.
And why would you conclude, from the existing recordings of BF speaking Spanish, that they all speak only one dialect of Spanish? Do all Spanish-speaking people speak only one dialect?
And as to your complete bafflement about where they learned Spanish, and from whom, I have some ideas that might help you with that. You are familiar with the stories of the indigenous peoples here on this continent, are you not? In particular, the stories about how some BF lived alongside them and were their trading partners, yes? So you see that it is not a huge leap to think that the BF might also have lived alongside the Spanish, when the Spanish arrived here in the 1400s. (They might not have lived openly alongside the Spanish, but they didn't always live openly alongside the indigenous peoples, either, and yet still somehow learned to speak their languages.)
So maybe -- just maybe -- the BF learned Spanish from Spanish-speaking people.