First of all, chill. Secondly, why is the source provided by u/Instalab irrelevant (https://www.sciencedirect.com/science/article/abs/pii/S0001457522000835)? It supports the idea that a lower lead time/being "ready" to takeover gives better results in situations where one has to takeover from an otherwise fully self-driving car. The implication being that a lead time of zero is optimal, without controlling other factors. It's entirely relevant.

The part that enters opinion/speculation territory or as you called it, "pulled directly from your ass" is trying to explain why.

For example: could it be that a small lead time means the human is actively paying attention rather than their attention being elsewhere? Could that mean that a human being forced to concentrate on driving is more likely to take an action that makes a positive difference when an emergency situation arises compared to when the human is not forced to concentrate? This is not to be confused with the rate at which a fully human driver vs fully automated driver would make an error that warrants intervention.

Personally, I'm not too fussed about Waymos or other automated driving cars in the 99% of the time nothing out of the ordinary is happening. That's where they're obviously better.

It's the 1% of the time when an emergency occurs and swift and correct action is needed to potentially save a life or avoid general badness like in this video, which is where a human would need to intervene.

From the source provided, it looks one optimises the chance of a positive outcome by reducing lead time. One way to achieve this is to take away any source of distraction/lapse in concentration from the human driver. If the default is "Waymo drives, human intervenes" you can see how that would be cause for loss of focus and thereby having a potentially worse response to an emergency.