+++ I would say the 40*20 FPS is more about throughput than just read out speed.
Nope... In order to throughout, sensor readout has to be completed first

For the record: I do not suggest that R5 will be a rolling shutter free camera,
Not sure what you are disagreeing with.
There is a time that's required to get the value from each pixel off the pixels. This time determines how noticeable the rolling shutter is. The A9 sets the record for consumer ILC in this regard with a value of ~ 1/160 s. The 1DX III according to the reports I've seen is closer to the more typical 1/60 s. If the R5 somehow gets closer to the A9 than the 1DX III in this, Canon withheld some major improvements from the 1DX III.
Throughput is not independent of this read speed, but there's more that goes into it. It may take the A9 II ~ 1/160 s to capture an image, but that doesn't mean it could take 160 FPS. The throughput is determined by the time of the additional steps to transport the data within the imaging pipeline and the actual processing times for the image and routines like AF, metering, tracking and so on.
So througput has an upper bound caused by the read speed. But they aren't the same.
I guess you're implying I'm using I'll suited words to describe these concepts. I use them because that's what makes sense to me and what I see them called commonly.