Right, but the crux of OIG's finding is that there isn't a hard difference between the two.

The plane was not built to the spec that FAA type-certified. But at the same time it wasn't clearly out of certification either.

There were significant changes made to MCAS since the the type certification. However, there was no clear delineation of what types of post-certification changes would require another FAA type certification.

The permeability of the two types of certifications and the amount of design changes that can happen between them is the entire problem OIG is pointing out.

> FAA essentially accepted proofs / was pressured to accept assertions from Boeing that the type was actually safe

No, the document says Boeing's ODA team was pressured to accept assertions from Boeing's management. The Boeing ODA team is employed by Boeing. They are given de facto self-certification authority either by actually self-certifying (in the airworthiness case), or by choosing what it flags to FAA for external certification (in the design/type case).