It wasn't originally my claim

Sorry, I wasn't paying attention and missed that. I apologize.

loads of modern computers don't use DDR5 or ECC variants of older generations at all, so don't have any error-correcting memory. If the wrong bit flips, they just crash.

Integrated memory ECC isn't the only check, it's an extra redundancy. The point of that paper was to show how often single bit errors occur within one part of a computer system.

memory errors are really rare

Right, because of redundancies. It takes 2 simultaneous bit flips in different regions of the memory in order to cause a memory error and it's still ~10% chance annually according to the paper I cited.