yeah, that rust-issue is definately a thing with these pistol-belts, that's gonna happen way too fast, so avoiding humidity on then as much as possible is strongly advised.
On the other hand a tiny bit of rust may be even quite accurate, since it happened to the real screen-used equipment as well, as can be told from closer looks to some specific images. Even the belts worn on the actual movieposter-shooting (which i guess happened somewhat before the actual filming with the gear/uniforms had even started) show some traces of rust around the pistol-belt eyelets. ...so
i'd say: worry a bit, but not too much.