It’s optimistic but it’s not gullible. Underlying the system is an anti-fraud measure. Each node entering transactions on the OVM has a bond. If it processes a fraudulent transaction its bond gets slashed. Not only that, though, but every node that adds a block to that fraudulent block also gets slashed. This gives everyone in the system an incentive to, first of all, not commit fraud and, second, to keep an eye on their peers (because the sooner fraud gets spotted, the less it costs the whole system).