The first, link icon is just that, a link to the post/comment the way you currently see it on the site. It's relative to the instance you have opened it on.

The second, fediverse icon is a little confusing to me, but it definitely isn't doing what I need it to. In my experience, it shows the link to the post/comment on OP's home instance if it's not the same as the instance you're browsing, even if they posted directly to this instance. My guess is that OP's home instance must create the post first, to then send it to the foreign instance, so technically, posts exist on both instances.
Long story short, the fediverse icon is relative to the OP.

In conclusion, neither of those buttons will open the post relative to my home instance, with the exception of some coincidences.