I'm pretty sure it was stated when they arrived that The Royal Guard combined are stronger than the rest of Soul Society put together... they are stupidly god tier don't even bother they're going to rape you anyway strong.
And they've been training Renji, Rukia Ichigo and possibly Byakuya, in addition to fixing their weapons and Gods only know what else.
Also, Renji was in shikai, with his blade extended after killing James, attacking a Sternritter who hadn't even released his Viollstandig, quite frankly if he HADN'T oneshot him I'd be confused as to what he was actually doing with the Royal Guard.