ELI5

Don't take my opinions too seriously, I'm just referencing my Astronomy notes (of which come from a single semester of a single class). With that said, here's my 2¢ guess:

I think they're trying to say that the last time the universe as a whole was 0°C, was probably before the formation of matter in the universe (I'll guess the inflation era, just after the birth of the universe, somewhere 10^(-35 to -33) seconds).

At this point in the universe, atoms cannot form (as the nuclear forces binding atoms are overwhelmed by the gravitational forces of all the energy in the Universe — you ever crush a cracker? It'd probably be like that). Perhaps even sub-atomic particles (protons, neutrons, etc.) can't form. All that's there is sub-sub-atomic particles (which we don't know much about, from my understanding).

So basically we (and the world, etc.) would be ripped apart at the sub-sub-atomic level by the immense forces (gravitational, etc. — remember, all of the matter/energy in the universe is being concentrated in a small place) of the early universe.

So, it's not that we would necessarily evaporate, nor that touching sub-sub-atomic matter would kill us, but more-so that we'd be crushed, at the sub-sub-atomic level, by the gravitational forces of the early universe. It'd probably be painless though, at least.