Platinum chests are awarded for
1) starting the raid
2) being in first place of the raid
3) being in second place of the raid

So far I've got ~2m PP with 62 divine and 114 demon souls for a total of 176 souls. With me getting MVP quite often, I must've gotten between 800 and 1000 tickets.
Yet so far I've only gotten 1 SSR wep from the gacha, completely unprecedented when compared with the previous wind and fire events from which I have 3* fire wep, 2* fire eido, 2* wind wep and 3* wind eido. And it should be taken into account that my previous raids didn't get me even 1m PP.

tl;dr - I'm fairly certain the drop rates of the gacha this time are lower by multiple times. Close to a thousand rolls should be enough to get a decent estimations of the % chance and so far it seems like it's around 0.1% from my experience.