The point is that although yes, the incumbent would be able to bring a claim, in the absence of a registered trade mark, a claim on the trade mark would be based on the established goodwill in the name. It would then be open to the alleged infringer to argue the incumbent lacked the necessary goodwill.
I went and read up a tiny bit and learned about the requirement of showing that goodwill has been taken advantage of. You are indeed correct, sorry to come off as argumentative. It "only" took me one back-and-forth before I went and checked my facts.
Hey, if you can't argue on a HN thread, when can you argue? In fairness it looks like protection for unregistered trade marks appears to differ on a state-by-state basis so the position really isn't that clear. Plus mere use appears to be sufficient to get a modicum of protection but on a very limited basis.
