Is ATX3.0/3.1 PSU backward compatible with the old H12SSL mob?

I’ve read that the ATX3.0/3.1 is supposed to be backwards compatible with old standard, yet, retailers of H12SSL claims the new PSU cannot be used on this old sever board, while a retailer of Seasonic claims the new PSU can support this mob.

Does anyone happens to have experience?

Many thanks,