I’m not an expert on the subject, but you can kind of force a sync by using the search option.

example

I just put the url from a part over there https://diyrpg.org/post/246 in the search box in our insurance and it started showing up here (without any of the comments)
Presumably, older comments could be synced manually with a link too, but that sounds like a lot of work and I haven’t tried it.
If there a specific comment you wanna interact with is not too bad, don’t this manually for all comments doesn’t sound fun.
I think there might be browser extensions to “open this in your preferred instance” but I haven’t tried any.

Silver lining is that stuff that happens after you first added the community should be more seamless.