Many restricted military technologies are fairly easy to detect. Nuclear weapons require a massive industrial input that has well known signatures, for example.
A robot is different. It can be something that's dual use. One day it's a regular robot. Look at the software. All civilian and a nice strong optics mount point on it.
Change the software and swap some of the camera gear for a small machine gun and use the rest for aiming, it's a killbot. This is just one example. Another obvious one is putting an autonomous drone software package into the flight computers of an airplane that can also be manned. This game can go on and on with just about any weapons system you can think of.
It doesn't take industrial facilities that are different from usual ones to make them. If you can make versatile robots for civilian use, and separately make weapons you just have to put them together at the last minute. They don't have any particular signatures the way chemical weapons and their precursors do. Most nations are already making ordinance, so who's to say whether a human is going to be in the loop to fire it or if it's triggered by an AI?
If people want to cheat on this, it'll be pretty easy to do so.
So far, the landmine bans haven't seemed to have slowed down the planting of them a bit in various wars. We have to have demining teams, not just for cleaning up old wars, but the very ones that are going on now.
I don't expect this to have a much greater effect.