That is entirely up to you. Of course, until every player has gigabit full-duplex secured fibre to the PC, with a FD provided UPS, and a FD provided PC, and a FD provided power company, and an FD provided watchman looking over your shoulder ready to shoot you if you even looked like you were going to hit the reset button, and absolutely massive server farms provided by FD to host everybody, and FD provided intercontinental fibre channels to link every pool of players together at acceptable latencies, all for free!
That has nothing to do with what he means. He's talking about Player A, B, and C.
Player A plays in Open, B in a PG, and C in Solo.
Player A, B and C all affect the BGS.
Player A, B and C all affect PP.
I think what he finds stupid is that; let's say Player B is backing Lavigny in PowerPlay, and Player A want's to stop Player B from expanding in Omicron Persei 8 (i know i know). He can't! Player A and B (and C for that matter) are never going to be in the same "shard" together so you can't attempt to stop people from invading systems etc.
In Powerplay you might get alerted that a system in the power you back is in Turmoil! due to other players invading it and undermining or something else equally naughty. If Player B and his PG are all "invading" this system there is nothing you can do to stop them.
It really is a mistake on FDs part imo. There should be different BGS's and Powerplay for each "shard" since you cannot impede other players from changing it.