I might be wrong here my friend but IRC, in DMM when the Rainbow Coins were introduced, there was a "roll back" / "back track" or how to call it.
Pretty much they added several months backwards to the day the mechanic was introduced and every single pull that yielded a dupe - thus giving you RC (rainbow coin), was refunded to you so to speak.
They did something similar with Nation battles just this last month. They reworked them completely, and you received all the flower gems from those stages even if you didnt complete them (those who did didnt get them again though).
While it is and argument to raise that "not everyone could do same amounto of pulls thus everone got the same equall number of RC", maybe, just maybe this is some promo/apoloRCoins to get it more flowing. Or maybe they were just lazy calculating the exact numbers for every player like they did in DMM and just tossed all of us a nice hefty sum of 500, giving us one and 1/3 of a rainbow
So yeah I might be wrong in my theory crafting here, but it certainly does seem to be something of that sort to me, especially knowing how these things usual work on DMM.
Alternatively - it might be some promo like what they did with the FB FKG collab, where we got that one free 6* of our choice.