How does roofer know the thickness of the sheathing without
ripping-off the shingles? Seems to me that he'll need to use the
same thickness to match the existing sheathing that has not buckled.

Probably from the experience of working on hundreds of roofs, and that there
is only one thickness used in your area. Probably half inch. And sometimes
a piece is visible on the outside that can be seen, so he doesn't need to go
inside. Ask him how he's sure. If you don't think the man knows what he's
doing, hire someone else.

Roofing isn't rocket science, and if there is a difference in the sheathing,
it's only going to be 1/8", and that can be shimmed so it is not a visible
transition.

Steve