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I didn't get around to turning it off. I did get around to testing it again, and, after a fashion, it does work!

The "Pre-AF" menu item does cause the camera to focus on it's own. To a point,  and if you are prepared to wait.

It has been turned on in my camera (a7iv) but, as I said, I hadn't observed such a benefit actually happening. This evening, I pointed the camera at an out-of-focus face. The viewfinder is showing, with a rectangle, that the face is recognised as a face. After three or four seconds, biff!,  the face comes into focus. I moved the camera from face to face, and each time (excepting when it got stuck on one face and wouldn't recognise another nearby one) the camera focused on that face. Each time, it took from one to several seconds to do so. And it is giving face rectangle: one has to press the shutter half way to refine that to eye focus. With the half-shutter press, you'd be right there, focussed, long before this pre-AF had even started to work.

Back to the question:  can the camera focus on its own?

Answer: Yes.

 But not, according to me, in a way that is much use. But there is probably someone, somewhere, who does find it useful.

Thanks for sparking my curiosity on this one. With a new camera (about 8 months old) one activated menu entries just because they seem like  a good idea. I did, and then forgot about it.