One can claim a trademark without registering it (the difference between ™ and ®). But, if one wanted to sue, you'd probably register it first. But, a claimed trademark that is suitably unique for your product is defensible if you can prove consistent usage pre-dating the new user of that mark.

I'd be pissed if someone took one of my open source projects, forked it, and also stole the name (and put "Open" in front, despite the fact that the thing they forked is Open Source), misleading users and diluting the brand with software I have no control over.

I don't even know what Warp is, but I'm mad as hell about it. As an Open Source developer of 30 years, I expect people to operate with something like honor and decency and respect for other people. Taking someone's open project and launching a competing fork with the same name is hugely disrespectful and dishonorable behavior.