[Brightcolours]

The 600mm is pretty much identical to the RF version. The 100mm/400mm are new designs.

The RF 600mm should indeed be identical to the RF version ;-)
But yes, the RF 600mm is optically identical to the EF version (EF 600mm f4 L IS USM III). The electronics, of course, are not.
Actually, the RF 400mm f2.8 IS USM is also "pretty much identical to the" EF version (EF 400mm f2.8 L IS USM III), optically. Again, the electronics are not, of course.
It is not strange that they chose to use the same optical designs, as both are top notch, very recent designs, and the shorter flange distance makes no difference with such tele prime lens design.

The 400mm/600mm look as if they just attached an adapter actually.

Indeed, which also makes sense (use the same barrel mouldings).

The RF 100mm f2.8 L Macro IS USM is (again) a pretty cool and interesting macro offering from Canon. Not only does it go to 1.4x magnification,
https://downloads.canon.com/nw/lens/rf10...ture-1.gif
it also offers spherical aberration control (which some might remember from the Nikkor AF 135mm f2 DC and Nikkor AF 105mm f2 DC), to be able to adjust bokeh.
https://cweb.canon.jp/eos/rf/lineup/rf10...fig-03.jpg