If I'm being honest, this is the kind of thing I would not have allowed. As you describe, your child can make it a few meters away from the side of the pool. Then what?
If your child gets tired, this will not help. This looks like it forced the child into a face-down position, and a tired child forced to be face-down in water doesn't seem like a great idea.
As a fellow parent of a 1.5 year old, I'm sure you've heard of latent drowning. [0] This device seems like it could promote the chances of that happening as well.
Looks reasonable as a support when your child is being taught how to swim (and is thus closely monitored). Looks absolutely horrible as a means of preventing your kid from drowning though.
disclaimer: I'm not in any way a professional when it comes to water safety, so this is just a remark; not advice.
Yea, it looks like a useful tool to provide support while following the warnings:
Warning: Not suitable for children under 36 months
Warning: Only to be used in water in which the child is within its depth and under adult supervision
Warning: To be used under the direct supervision of an adult
I don't think it's a mistake that "under adult supervision" is said twice.
I assume that they are good as he uses them in his swimming classes and with it he can swim a few metres to the side of the pool.