> If its just a sub page, filter etc, then i think in many cases it should only swap into the dom without updating the history.

It depends. If it's a 'resource' (in the REST sense) then it should actually push the URL of the resource into history, because the URL should correspond to the currently viewed resource. This is exactly what I was talking about earlier, it's super important as a basic hypermedia principle.

> if you use hx-boost to swap in fragments, then the URL that gets updated/saved in history wouldn't actually load the same page if you loaded it from a bookmark

See amanzi's explanation or my blog post where I explain the same thing. With htmx we can easily check for the presence of a request header in the backend and serve the appropriate version of the resource: either a partial (fragment) rendering or a full page that contains the resource. I highly recommend reading my blog post: it's not a huge commitment and it will clarify these issues for you.