Also, I believe that if Apple is working on blood glucose estimation, it's probably not the way described in this article.

I could be wrong that Apple will do it this way, but the current hardware supports different wavelengths PPG, (as well as the Samsung Watch Galaxy 8), which might be able to use to estimate blood glucose variations. But it's pretty complex, especially to generalize.

I wouldn't be surprised if they start like blood pressure, just estimations as to how it's going up and down.

I know that because I'm literally working on this with https://trackourhearts.com, using an AI model I created to estimate blood glucose, though I'm doing it in a personalized way so I can provide more absolute estimates but it's still incredibly hard from PPG as the wavelengths available aren't the most optimized for this.