https://en.wikipedia.org/wiki/Disney_utilidor_system

so I suspect that the tunnels are limited if they exist at all.

They're not top secret, but you've guessed right all along.

It's more of a mixture of tunnels, backstage corridors between attractions, and open areas within internal attraction-voids.

As in, there really are tunnels, but no one with any knowledge on the subject never claimed the whole thing was tunnels.

EDIT: Actually, you're both right in parts because in large portions also have the whole elevated by one level thing, and the "outside" is really the second story. I skimmed that critical part on the first look at the wiki.