That must be true. That must be true. The numbers one through four must exist because there are four different words. So addition must be true, because you add those units up and get the number four. The alphabet exists.
Parts of speech exist, like nouns and verbs.
They must, in fact, be necessarily true, given the statement itself. If there's such a statement uttered in English, then all these other things must be true. It's impossible for them not to be true. That's why radical skepticism like this is not justified. We should take the burden of proof to defend our skepticism instead of simply asserting our skepticism.