Good information.
Some of the information in the article is contained on the spec sheet for the R5 and R3.
Everything I have found so far is Pre RF 600mm F/4, and nothing much about it.

I just ran through a few more non scientific hand held tests. I am getting right around 5 stops of IS (a little more and a little less on each try) with the RF 600mm.
I tested the EF 500mm II as well, as advertised I am getting about 4 stops.