Do you consider Crionics to be a thrash metal song? I don't know how someone can listen to it, Transylvania, and Spiral Dream and not hear that they all have much more in common with each other than they do with, say, anything Exodus put on their first four albums. Or again, Orphan Gypsy and When Angels Die.
And obviously I'm not saying you haven't heard Iron Maiden, but listening to the general progression of USPM in the early/mid 80s makes it easier to hear the similarities. You hated Omen in one of the song rating threads, right? You'd probably also say I'm crazy to say that Omen and early Slayer had a shit ton in common, even though riff-wise it's obviously the case. Coroner's RIP is built largely on the ideas that define USPM.