Interesting so it seems like this infers high blood pressure from other signals rather than actually checking it directly? The linked article is light on technical detail, I wonder how they’re detecting it. I thought for a moment that they had added blood pressure monitoring to the watch, which would be awesome if it was possible, but that doesn’t seem to be the case.

Yeah -- it uses deep learning on PPG (heart rate signals). It's most likely picking up on the shape of the pulse wave, pulse wave velocity, and other proxies to blood pressure. It's worth noting that the Apple Watch doesn't give you a numerical blood pressure reading, just an alert if it thinks your blood pressure is high on average.