That doesn't match with what we know about Rosetta 2. Rosetta 2 can't run processes with mixed architectures, so ARM hosts can only run in-process ARM plug-ins,

That doesn't match with what we know about Rosetta 2. Rosetta 2 can't run processes with mixed architectures, so ARM hosts can only run in-process ARM plug-ins, and x86 hosts can only run in-process x86 plug-ins. Apple's AUv3 plug-in architecture is out-of-process, so you can mix those, but there is no way you can mix ARM and x86 VSTs, for example, without specific work by hosts to provide an out-of-process shim translation layer. Either he's talking about AUv3 specifically, or the hosts he tested already are doing out-of-process wrapping, or Rosetta 2 is actually magic (AFAICT this isn't a generally solvable problem at that layer), or he's confused.