interesting question...
to me, it's like asking "How much do you need to know before you can learn something new?"

Every baby ever born started from zero and learned from there.

No matter what you're learning, if you start with the understanding that you don't know it yet, but you can learn with some effort and without giving up... you will make progress. And that's all learning is: progress. There is no final destination to learning.

What makes BIAB uniquely usable in my opinion is that even with no experience or knowledge of music at all, an inquisitive person can go through the demo files, replace a chord here and there, and come up with something new.

On the other end of the scale, those with a lifetime of acquired musical knowledge will find multitudes of theory-based tools for developing their music ideas in BIAB

Whereas a novice might use a hammer to fix a loose board, a carpenter might use it to build a whole house. So, the answer to the question "What's the entry level knowledge required to use a hammer?" would be quite different than the answer to The question "what's the entry level knowledge required to build a house?"

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0