I assume a person like Back is familiar enough with opsec that he could intentionally change his writing style, and also "accidentally" leak his IP, (via a VPN), when he is somewhere else in the world.
He would have also had to mimic another cryptocurrency researcher's stylometry that ended up matching far better. Which do you honestly think happened:
A: Back is Satoshi, mimics X (who has many other reasons to be Satoshi) for hundreds of emails.
B: Back isn't Satoshi, X's stylometry matches Satoshi.
A: Back is Satoshi, mimics X (who has many other reasons to be Satoshi) for hundreds of emails.
B: Back isn't Satoshi, X's stylometry matches Satoshi.