I happen to be lurking on this forum which is convenient. I own the Google code that parses that.
The linked thread is a good answer to the comment tangent. I’ll address the rest here.
It’s essentially non standard to interpret HTML id attributes as node IDs. It was done in the very beginning of Google’s microdata parsing probably for fuzzy reasons. You are supposed to use itemid if you want to do that explicitly. I hope to remove that hack someday but it’s hard to pull something like that out without losses.
Secondly, IRIs do not have to be dereferencable. That’s a suggestion from W3C but many IRIs are not and Google definitely doesn’t require it.
This is only a problem if it causes nodes in the structured data to inadvertently merge like if you used an itemid of the same value somewhere else in the HTML. Otherwise it’s just a weirdness that can be ignored.
Oh, and please don’t switch to JSON-LD. Honestly that’s preferred for text heavy markup like forums. Having to duplicate the textual contents is silly. It’s just easier to author which is why we’ve been pushing it.