Yes that is kinda how Segwit works.
However you missed the part for BCH.
They put the miner claim into the protocol.
The attack was a miner using the said code to take the coin for themselves. The code allowed it. However it was not "right" so the 2 pools roll backed the tx. This is the question it sounds alright but dangerous precedent for them. It wasn't so much a bug but ethics.