Why would you need something like that? What circumstance could it handle that an ATS can't?
I think he is saying that, for whatever reason, he does not have a place to connect the PV on the downstream side of the ATS and so wants to add a separate relay to isolate the PV from the generator.
Since there is no possible interference with the operation of the ATS to isolate building wiring from POCO while on generator power, I do not see any problem with this.
If there were a place to make a supply side connection AND the AHJ and POCO allow supply side connections, that would be logically simpler.