Those are called "water wings" in the US and the person you were replying to feels quite the opposite about their safety; the ones I've seen available for purchase in the states are easy to dislodge accidentally.
The ones which are just placed on the arms and not tethered to each other have a failure mode where they easily come off if the child puts their arms straight up. Unfortunately, this is also a common drowning fear response.
In general, I want people to have full market freedoms, but I put those water wings pretty near lawn darts in terms of danger.
I'd call lawn darts safer, as the danger with them is a lot more obvious. Everyone understands that throwing sharp things at people will lead to injury. It takes a significantly more informed consumer to know that a product masquerading as a safety tool is ineffectual at best.
All of the ones I've encountered in Europe have a butterfly-ish creature on one side and warnings in a dozen languages on the other side - not a safe flotation device.
In general, I want people to have full market freedoms, but I put those water wings pretty near lawn darts in terms of danger.