I can think of one possible explanation for the "exacto blades" tyre arrangement originally shown.
Banksy is a street artist, a guy who is self made and who likes to do hands-on art, so perhaps he did start with a completely self made shredder? He probably wouldn't initially have access to, or the direct resources for the CNC machined parts the eventual system required (knurled rollers etc) so i suggest that he started to experiment with a very DIY shredder, using cutting blades, which he would have in his studio, and an old motor (looks like an wheelchair motor to me). However, as the power and loads to reliably shred a thick canvas are considerable, i'm going to suggest that after some early trials (hence all the different system parts (belts/motors/rollers etc) shown, he eventually had to call in help, from a more professional engineering company (plenty of small ones exist in the uk, doing public art/displays and similar things (hello mikeselectricstuff!) and they refined the design to the eventual system.
For Banksy though, i could see that he'd very much want to make it look like he did it all, and not that he had to call in outside (possibly commercial, ie paid) help, hence all the smoke and mirrors releasing of footage of the earlier DIY systems?