Isn't Promos just technically an RNG- "Threshold"?
Like, in the back-end I'd say it works like this:
You get to 1000 ELO (just a random number here)
and based on whether you get to 1075 (+/- 25 Elo per match) in the next 5 matches decides whether you get to call yourself "Plat"
or whether you are going to lose all the Elo collected during these 5 games and more than that.
That basically sounds like Poker to me. Especially if your Winrate is 51% or something even lower, the chance that 3 of these 5 games you are going to play are the winning ones is so low, despite you winning (with a tiny margin yes) more than you lose.
Well, thinking about it this way, it sort of requires you to be an excellent player in your current division to climb up, to make sure you can stuff 3 winning games in there. Seeing it from this perspective makes sense again.