They ARE saying it's being wielded by a staunch believer. 'The enchanted Mjolnir was once a religious relic, and wielded by a staunch believer it has a profound effect... as the monster bursts to flame and ashens into dust'. I'd call that the effect of a staunch believer, wouldn't you? It's not saying that Thor's not a believer, it's just saying that Mjolnir is no longer technically a 'relic'.