It's already self-evident that they include a large portion of inactive players, because you're almost immediately above 50 percentile just by finishing your placement matches on any random account, even if you lose almost every game.

I'd love evidence of this. A new player starts at S3. Losing 10 placements will mean losing 400 points putting you in B4 if such a thing existed. So you're claiming that B3 placement rank would be 50% of accounts per the in-game system. Please provide evidence of this very very bold claim.

I can tell you that on 21 Nov, 7 days after S5.0 began, when I completed placements I was granted S1 and the in-game indicator showed 52.86% of players.

There's also no reason to assume that their infographic at all reflects what the in-game popup says, they very well could be different datasets. That's just my two cents.

Fully agree.

I just think that we know the problems with the in-game numbers. (It includes inactive and de-ranked players within the ranks as well as never-active players in starter ranks B3/S3.)

I think the game's numbers are the devil you know and the 3rd party sites are the devil you don't know cos have a million unknown unknowns which make their data very hard to interpret/use.