By Bb- natural I assume you mean Bb-C-Db-Eb-F-Gb-Ab-Bb?
I think this illustrates the problem with learning the modes as the Xth mode of major key Y. I don't think this example has much if anything to do with Ab, and you don't need to appeal to Ab.
You have to remember that the basic reason for playing anything is that it sounds the way you want it to sound. Theory is just a tool for explaining how some things work, not for telling you what sounds good.
But all that aside, I can think of a couple possible theoretical reasons for what he played. One is simply to create a certain kind of strong pull back down to the F. When you have a minor iv chord, you have two tones (Db and Bb) that are half steps above the nearest notes in the tonic chord. In certain contexts, those tones will often sound like they want to resolve downward (to C and A). That's a fairly strong movement. Also, the Gb (in the Bb natural minor scale) may feel like it wants to go down to the F. Using a minor iv in a major key yields a different effect from using the expected major IV.
The guy may also have been intending to suggest a couple of the tones you hear in altered scales--such as the flat 9 (Gb) and the sharp 9 (Ab/G#)--though the other notes don't quite fit with that.
You could also imagine that the overall "blanket" tonality is going from F ionian to F phrygian and back to F ionian. Several ways to skin this cat. But the basic reason he's playing this is that he likes how it sounds.
