The SPEs on the cell processor are actually pretty good at rendering graphics. In a lot of late gen exclusive PS3 games you can see that the developers utilised them more and more for graphics rendering. So the plan was to have the SPEs on both cell processors do all the graphics.
Stenzek gets a ton of abuse from the emulation community that is undeserved. I remember when he made PlayStation 2 emulation on Android possible with AetherSX2 under another username/alias, a massive technological leap, and the community treated him like trash. Moves like this are just in response to the entitlement and poor behaviour that some people directed towards Stenzek. Yes it sucks for the rest of us who behave appropriately online, but none of this would be happening if others treated the guy with respect in the first place.