How would they know if he had it back then or not...
Until it's confirmed we don't know how PS is unlocked. I also think that you need EMS not Rinnegan, but it's not confirmed.
dude.....did you not see where Tsunade says "to think that my grandfather fought something like this"
That means that Hashi fought PS
Madara only had EMS when he fought Hashi
Madara didn't get the rinnegan until he was old, and only after he inserted himself with Hashi cells
Madara didn't get Hashi cells until after his final battle with Hashi
hence Madara had PS when he had EMS....end of discussion