It is usually considered a form of mechanical media...
Bacteria will grow on just about any solid surface, which definitely includes polyfill...
The problem you will encounter considering polyfill as a bio media is it will collect a lot of waste and clog up. At that point the natural thing to do is discard it and replace it with new. Bio Media not supposed to be discarded like this.
You could stuff a canister with polyfill and regularly remove & discard only the nasty stuff closest to the intake keepign that which is deeper in the filter to collect & cultivate bacteria.