I don't disagree with your assessment, and to be honest I can't think of many times that this has happened in a game. Normally, you would send a noble to a besieged city to attack the aggressor and lift the siege. I can see that perhaps you might move them there and be waiting for another noble to join them later in that game round? In which case the noble at the location is considered besieged and therefore cannot accept troops from a mercenary draft in the meantime?
To be honest I think this ruling is to keep things simple and consistent although if you prefer, you can consider that the army is camped outside the siege waiting to strike. After all, if the defending noble tries to break the siege they get no support from the city defenses.
I can't see how you could disagree with my assessment other than just arguing that it makes things easier from a gameplay point of view. You would not be able to sneak an army into a besieged city under the noses of the besiegers.
I will have to figure out the best way to play this but will probably mark the Noble moving into the area in some way as being separate and outside of the city. No transfers of forces between Nobles; field battle if the besieging side wants to attack this new Noble outside the city.