I never saw any convincing argument that AMP helped pushed a walled garden, at least not in any common accepted sense of that term.

It was open source, it was used by many others including competitors, it was optional and it didn't block access from anyone. Having an AMP version in no way "locked" you to any garden, AMP versions aren't even meant to be the canonical page anyways.

It may have had a lot of issues, but "walled garden" would not be one of them.