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.
Satoshi also envisioned that miners would attack the contentious fork with the shortest chain and least PoW and kill it off. But that didn't happen. Because he didn't anticipate that mega miners would contentiously fork and collude to keep said fork alive (along side the main chain), and for selfish or malicious reasons. And then brokers and exchanges decide to help keep it alive as well.
Once this happened, all of the dogma of Satoshi's vision in his whitepaper got nullified. All bets are off, Katie bar the door, the end users now have the power to decide what is Bitcoin. Mega miners have fucked themselves as potential bad actors in the system. Brokers and exchanges have also now made themselves suspect.