Climbing in rank seems to be more about well-timed win streaks than overall winrate. You can win every game in a division from 0 LP to 100 LP (let's say 5 games at 20 LP each) and then lose your first two promo matches and not get promoted. That's 5 out of 7 games, or a 71.4% winrate. A great winrate, but you don't get promoted because you didn't win enough games consecutively at the right time.
Alternatively, you can bounce around winning and losing games in a division and if you're getting slightly more LP per win than what you're losing per loss and possibly also having some losses "absorbed" by your MMR when you're at 0 LP already, you can theoretically advance with a sub-50% winrate over a large number of games, just because you hit your streak at the right time (ie you hit a 5 game streak around ~50 LP, which gives you the three wins you need to get into promos and the two needed to win them).
I wouldn't say that winrate is entirely meaningless, though, as a good winrate will at least give you more opportunities to be in striking distance to hit the win streak required to advanced in ranked. I think promos are too reliant on luck, though.
