> I'm not sure that's relevant for privacy - for privacy, the specific thing you need is content blocking, right?

Yes, and for example (to my knowledge) there is no good granular script blocker (e.g. umatrix, ublock origin, noscript) in any browser available for iphone because there is no extension support in browsers like Chrome and Firefox.

Not really. If your rendering engines does the spying for you than blocking other tracking and adware still leaves you without real privacy. Coupled with a lack of fingerprinting protection, simple content blocking may not even be enough to reasonably protect you from third party trackers.

Is the rendering engine doing the spying for you?

This article seems like it's focusing on websites spying on you, and I don't think anyone (either this article or elsewhere) has made a serious allegation that WebKit does actually spy on you. It would be pretty newsworthy if it did!