I am thinking that Core not supporting SegWit2x may be because after SegWit is activated, all eyes will be on them to raise the block size to 2mb. By not supporting it they can push for just moving on with SegWit and LN instead. They are also balking at the 3 month timeline which is not a lot of time for testing something that would fork the chain.

I can also see that the miners are agreeing to this compromise because they see that BIP148 will probably get SegWit activated so they might as well try to get the 2mb if they can.

Either way, it's looking like we will have SegWit some time around August.