I haven't seen any confirmation that the TCs are RF-RF. It makes a lot of sense them to be RF-EF to avoid stacked adaptors for RF-EF plus EF TC 1.4 or 2x to use current EF big whites.Sorry that’s wrong.
The new teleconverters being released are native RF.
If the new TCs are RF-RF then it is somewhat confusing for the aperture for the new RF "big black" (I claim copyright for this one!) lenses
A previous thread outlines the valid use cases for 600/800 f11 new lens but it is less convincing when combined with the TCs if they are RF-RF
Canon RF 100-500mm f/4-7.1L IS USM -> 700mm @f10 (1.4x) and 1000mm @ ~f14 (2x)
Canon RF 600mm f/11 DO IS STM -> 840mm @f16 (1.4x) and 1200mm @ f22 (2x)
Canon RF 800mm f/11 DO IS STM -> 1120mm @f16 (1.4x) and 1600mm @ f22 (2x)
I imagine that Canon would prefer people buy all three lenses rather than use any of them with TCs. It isn't clear if AF would work wide-open @f22!