Yes, you are of course correct, but they shouldnīt be
that restrictive. I mean how complicated are these gestures that a wizard canīt even wear a light armor while casting spells. Do you need to be a full fledged acrobat to use magic?
On the penalty to thieving skills while using armor issue, IIRC there is something written in the manual about it, though I havenīt checked if itīs used in the game. The restrictions on the kind of armor a plain thief can use canīt be too big and he would have high enough in his skills to counter any potential penalty.