They can't ban the account. The account would be the downloader's and presumably, would auto-generate unfollow actions just as a user in a browser would manually. If they break the script, they'll probably break the unfollow UI for legitimate human user's and create evidence that they employ dark UI practices as a core part of their business strategy, which would be a P.R. nightmare.
If they break the script, they'll probably break the unfollow UI for legitimate human users
Messing with this is a lose-lose for Facebook.
Nah, this sort of thing really happens all the time. Think hundreds of scripts like this automatically disabled each day. This one just flukily got some press attention. Large tech companies will have teams entirely dedicated to preventing scripts from doing things while keeping the site running as normal for regular users.
Messing with this is a lose-lose for Facebook.