This is ridiculous. The concept of the "longest chain" only has a meaning considering the same difficulty. When you change that (at that point we are talking about a completely different thing longest or not) you can easily get a longer chain with ANY PoW you want. That's exactly what Bcash did, and now it doesn't matter how long they get, because they are using a lower diffculty and... a smaller accumulated proof of work.
It's obvious that Satoshi thought about all this when he repeated the definition several times, including: "The longest chain which has the greatest proof-of-work effort invested in it" in section 4.
He had hard-forks in consideration, so the only thing that he thought of worthy differentiating between forks is what he stated above. There's no other way to be sure which chain is Bitcoin.