You are assuming the number of SegWit2x supporting nodes will remain as low as they are now. We are still months away from a coinsplit. We will start to see SegWit2x node counts rise as the split gets nearer. If you want to stay off the chain with the longest work, you’ll have to run a client designed to do explicitly that. If Bitcoin were as easy to Sybil attack as you are claiming, it would be dead already.