Photoplethysmogram devices like this measure blood volume. That works great for your heart rate. It is pretty terrible for your blood pressure. In theory they measure the speed an artery expands as blood passes.
1. Since they can't measure blood flow, only volume, they can't tell if your BP is low or if they just aren't centered on an artery. They can't measure diastolic, only the difference between systolic and diastolic.
2. Since they don't know the flow, they can't even guess the size of the artery, so they're basically just guessing how much it should be expanding. They also have to know how tall you are to even get close.
3. They measure the derivative of the increase in volume to try to correlate that with blood pressure. That's very variable with the specific artery and the condition of the artery- there's a lot of (primarily age) variance in artery elasticity.
Papers indicating accuracy should be subject to high scrutiny. The device basically starts out by guessing that you're healthy and normal for your age and height. If you take a bunch of people that are healthy for their age and height (most people), the device will perform very well. It will also be practically useless.
If you have ideopathic large swings in blood pressure, like POTS, or fainting, or migraines, a device like this can give you a bit of warning. It will not give you accurate readings or track long-term changes, and those issues are made much worse because of opaque calibrations etc.
Do you think the EU regulatory agencies haven't properly accounted for the claims made by the makers of this device?
It looks very promising and it is clinically approved.
Your theory craft seems to check out, but they could have proprietary technology that Huawei is yet to clone/rip-off, so it's not a well-known methodology. Funnily enough, they actually tested some of their algorithms in China based on their papers, so maybe it is just a matter of time.
That being said, if you have industry inside knowledge, that's a little different.
The technology already exists and is approved by the EU.
https://aktiia.com/