I'ma see if I can make this as clear as possible

Riot made it so your visible rank changes slower.

Riot did not change your mmr gains/losses.

This is to combat smurfing because boosters and content creators do so with an eye on efficiency. The longer it takes to make a Diamond account, the less efficient they are. If they're smurfing, they'll hit Diamond games way sooner than their actual visible rank reflects, so this change impacts normal players less than legit high Diamond+ level players that boost accounts.

If you're climbing at more normal rates, the changes Riot made have less of an impact on you because you'll occasionally lose, which will bring your mmr closer to your visible rank every so often. If you're a smurf, you'll be in Plat games on a Gold account and Diamond games on a Plat account, making it harder to flip accounts and sell them for money or make YT content off of "smurfing my way to Diamond for the 37th time". That's the intention of this change.