A ''Haswell Ready'' PSU will work with a Ryzen 5 2600x?

Haswell introduced a very lower power sleep state that effected several PSU's, esp if the PSU had a larger load on the non-primary rails (3.3v &5v) then the 12v rail could go out of spec. This just means that the PSU is ready for Haswell and newer processors that support c7 sleep states.
 


It will work. As popatim said it only means it supports very low power modes the CPU can enter when it's asleep. Those were completely redone with Haswell vs previous generation CPUs. Kaby is newer than Haswell but as far as I know the sleep state requirements didn't change.