No, they just focus on power consumption more than the other browsers (they also have the advantage of not needing to work cross-platform, making this easier). This is often why Safari is behind other browsers on features: a focus on quality of implementation of quantity of features.
I also would want a source for that. At least in the mobile dev space, it has always been common practice for them to leverage private APIs to do things regular app devs cant.
> At least in the mobile dev space, it has always been common practice for them to leverage private APIs to do things regular app devs cant.
On mobile (iOS), yes. But power consumption comparisons tend to be on desktop (macOS) because the other browsers can't run on iOS at all. And I don't think Apple can enforce that API privacy on macOS because they unlike iOS they allow apps to be installed from outside the App Store.
(I use Firefox anyway, because I like its features and I'm a zealot too, but if on battery without a charger nearby, I use Safari.)