That’s not true tho. I mean, it’s true that riot’s matchmaker has been broken since PC season 1, yes.
The thing that you are missing is that statistically, over time, your MMR and your win-rate equal out over time. This means that over time, your MMR and your supposed “rank” can diverge, and you can get legit “hard-stuck” because the matchmaker is efficient at setting you a 50/50 win-rate.
Improving your skills will only net you a few wins before the MMR adjusts (because MMR adjusts far more quickly than raising rank), causing you to get back to 50/50.
There are ways to “beat the matchmaker” such as duo-and trio-queue, or being better than the entire available pool of players (I.e., being a Master level player in Plat), but otherwise, the way to climb ranks is via collecting Fortitude points (while you are under Diamond).
After Diamond, your MMR and Rank progress get linked by LP points, so the problem is lessened.
TLDR: you can improve your MMR via skill, but that won’t improve your rank below Diamond.