That's not the same at all. PLA-printed 3D prints don't take 1000s of years to break down, but they're very clearly not something you add to the composter.
It is though? Sure, you can't just throw it on a compost pile and wait a few weeks for it to rot away. That's why leather is processed (tanned) in the first place, otherwise it would be a pretty useless material.
But it will biodegrade. In a few years instead of thousands of years like plastics.
It depends on the process. Some processes literally make leather non-biodegradable. I'm not saying that faux leather is any better I'm just saying it's more complicated than people realise. The leather industry could certainly use some improvement.