Seat supports shall be of the type listed on FIA technical list No.12 or
No. 40 (lateral, bottom, etc)
The writing isn't totally clear, but I read that section to refer to "Seats homologated to and mounted in accordance with FIA standard 8855-1999", rather than all seats. For instance, I don't think anyone has postulated that all seat back braces must conform to an FIA standard, have they? By your reading, that would be the case.