It checks that every block header commits to the previous block header, and that the assumevalid block is in the header chain.

Isn't this the same level of security as SPV?

It also rebuilds the utxo set from scratch

But if they don't check for validity, the node is trusting that the rest of the network did such checks. What is the difference between this and an SPV node trusting that the longest the majority of hash power isn't tricking them?