The title is intentionally misleading because the authors know all of mathematics is not possible to formalize, strictly speaking. And strict meaning is what math is all about.
Would you have been happier if we had called it "...all known mathematics"? This is what we are plainly talking about. The question we attempt to address is how best to digitise the corpus of known mathematics.
You cannot formalize ALL of mathematics. We have known that for a while. Hence my point about the title being clickbait.