essentially it boills down to the problem that flipping a few bits of the transaction let you pass off a fake transaction as the one that matches a signature, and this is why it is a bad thing for lightning, as this essentially can mean the difference between a truthful record of the channel state changes and me robbing you

with schnorr signatures on the transactions you can't twiddle those bits on the edge, segwit is a countermeasure that only applies to ECDSA signature algorithm