When first looking to buy a television, I almost fell for that blatant false advertising scheme. It shouldn't be allowed. They get away with it because the television will decode the HDR signal, and the television tells the hardware that, which is why you can choose to turn HDR on, but yeah...it isn't HDR, and you're absolutely spot on that you need to research heavily before buying a television because of it.
I assume this is why many people claim that HDR is just a marketing gimmick and not an actual benefit, but anyone who has an actual HDR television knows the great benefit HDR is for image.