They are a con, there are no extant non-research devices that can monitor blood glucose directly from the wrist.
There is talk of Apple (or rather one of their contractors) developing something that will fit in a future Apple Watch, though the consensus is, as far as I understand, that it's at the very least one watch generation away and I've not seen anything specific about the technology they are hoping to use.
The only thing that looks like it's nearly ready for commercial use/availability is this, though it is still an R&D device and is not available for purchase as far as I understand (thought the web page looks like an sales pitch!): [I can't post links, but do a search for "AFON technology glucowear"]
Or perhaps the adverts are bending the truth a little? There are lots of smart watches that can relay blood glucose from e.g. a Libre sensor - for example my Garmin watch displays my blood glucose with my phone acting as the intermediary and feeding it the value from my Libre sensor.
