Thank you so much, that exactly answers my question with the official response (that guy works at Meta) that confirms it’s the same base model!
I was concerned primarily because in the release notes it strangely didn’t mention it anywhere, and I thought it would have been important enough to mention.
You are right. Their description of “SOTA Open Source TTS” caused me to assume it was open source, but it’s clear that
So, it’s “source available” and not released under a permissive licence.