Practically what's gonna happen (imo) - if you log more than 1 account from same IP you get on the watchlist of CCP anti-cheat watchdog sub-routine. If the server records simultaneous input originating from clients on the watchlist, it will flag the event which will (hopefully) be reviewed and action taken. I inserted "hopefully" because few weeks ago I got banned by Trion for boxing Archeage even though I play Rift and Archeage was not installed on my PC - which smelled like computer-generated auto-response (my ban was lifted only after I wrote to a senior GM). Archeage and Rift share same launcher, meaning that most likely I got flagged just by logging in from same IP.

Thus, I think this new preventive (but not obligatory) feature in ISBoxer is cool.