As the lenses seems to be no other than EF with an included internal adapter there should be no problem with a lens too close at the back. The first 2cm should be totally empty.
Or am I wrong?
They are compatible, without issue. The confusion probably comes from the fact that on the RF extender pages on the Canon USA store page, the Compatibility section lists only the RF 100-500, 600/11 and 800/11.
On the main product page, the RF 400/2.8 and 600/4 are included on the list.
Since this is a thread on lens availability, I’ll point out (again) that the Canon product pages and the Canon store pages do not seem to be in sync. With many hard-to-find RF lenses, they show as in stock on the product page but when you go to the store and add them to your cart, they show as backordered.