Here is a translated quote from a Russian Canon rep (not some random person on the Internet as you say):
" Canon is aiming this camera at professionals and advanced amateurs. The
Canon EOS R3 is not intended to replace the Canon EOS-1D X Mark III.
The camera will be intended for wildlife, sports, racing, and photojournalism."
It seems to me that his two sentences contradict each other. Aren't those uses exactly what the 1Dx3 is for? So how is not a replacement to the 1Dx3 if it is faster, most likely higher resolution, better autofocus, has an RF mount, does have a built in grip like the 1Dx3, probably better video given that it's mirrorless. It's true that we don't know the buffer size, but I imagine it will be substantial if it can pump out 30fps.
Here is a quote from Canon's own development announcement:
The camera body will be entirely new and accentuates the camera’s high-performance design. It’s a one-piece design, integrating the body with a vertical grip section. The weather and dust-resistance will be equivalent to that of EOS-1D class cameras — an essential consideration for nature, wildlife, sports and photojournalism content creators working in extreme conditions.
Again, based on the info we know now, how will the R3 be inferior to the 1Dx3? Even if the 1Dx3 has a larger buffer, the R3 still seems to beat it in most other areas. Technology marches on. And the earth is round.