I'm with you. Replace "the external world exists" with some logical tautology such as De Morgan's Law (not (A and B) <=> not A or not B). Formalism has all sorts of gotchas, so I'm going to assert that it's possible to be skeptical of De Morgan's Law being a tautology...

...which - as the author describes - implies I can go down that infinite spiral of regressive speculation. But this is not the reason that De Morgan's Law is a tautology.

Even worse, the author's argument really let's us assert anything, doesn't it? I've read the paper twice and I can't see any protections from using this argument to assert a contradiction.