From my understanding scraping a public website you'd otherwise have access to is legal in the US. However, re-hosting that scrapped copyright content, particularly if it isn't transformative/fair-use (inc. only partial content) is very much either violates copyright, or best case is legally a gray area.

Google got dragged into court for exactly this, and they were summarizing/snipping, so they had a fair-use argument to make. This is a 1:1 reproduction of the content, so the legal arguments on fair-use would be harder to make I suspect.

I've personally used XCancel, and I really like it, but stepping back legally they're on shaky ground if Twitter decides to stop it.

AFAIK, to date Twitter hasn't required its users agree to a copyright assignment or an exclusive license to the content of their tweets, so it wouldn't be a copyright suit. Twitter wouldn't have standing to bring suit, as that would be reserved for the actual copyright holders, i.e. Twitter's userbase.