Yes, it's just html according to their guide (
https://bsky.social/about/blog/post-embeds-guide), which offers the helpful workaround for sites that don't support it, like Substack, of "taking a screenshot of Bluesky posts and linking the post URL instead." It should technically be possible for XenForo, not sure if anyone's implemented it yet though.
There is also a workaround, where you put a 'c' in front of 'bsky.app', e.g. with
@St. Widge's Underhill post:
https://cbsky.app/profile/nickunderhill.bsky.social/post/3lahjzotsrc2c
But clearly it only kind of works, in that it's displaying it, but not as it would be on the site/app. I think it's a third-party site that basically proxies the content, same as fxtwitter.com/fixupx.com, to allow embedding posts where it otherwise doesn't work (like on Discord).