I don't think this is especially true. And I don't think the rules for swimming do anything to support balance. They're just clunky and incomplete, and disconnected from other relevant elements of the game like STR and CON as ability scores.
In fact it is to me far more arguable if anyone would have a better backstab, it would instead be an assassin type Prof, like ninja. The rest of the thief's skills boil down to fairly mundane abilities, henceforth the question.
I would expect that being stabbed in the back by a knight, or really anyone who is handy with a blade, to be a pretty unhappy experience. The D&D tradition of backstab/sneak attack is a mechanical device, bound up with the class build rules and the to hit/hit point rules.
As far as mundane abilities are concerned, I don't see why sneaking and climbing are any more mundane than (say) fighting. They're all things that people can have a natural aptitude for, and get better at by practising them.
