Segwit2x was an initiative to increase the block size. It only required replacing Bitcoin Core because Bitcoin Core refused to merge a block size increase. If they had agreed to merge a block size increase, then btc1 would not have been necessary.
The Bitcoin developers (not "Bitcoin Core") had just implemented a block size increase after years of discussion.
The segwit2x intiative didn't take part in this. They didn't bother even writing a specification, let alone a proposal or take it through public discussion.