Yes, that's true for propagators. Those newer players (non-propagators) who don't have a rating based on at least 8 rounds yet are not part of the fixed ratings pool and can shoot as high of a rating as possible without taking points from others.
This fixed pool of propagator ratings at a league or tournament has been a fundamental basis of the system from the beginning. Think about an isolated island of players. If the only people they ever play are each other, then it doesn't matter if some players are taking rating points from other players because the system simply shows how much better some players are getting in comparison to the other players on the island. It doesn't matter how their ratings compare with players located anywhere else in the world since they aren't playing them.
Now if some of those players play in events outside the island, they will either gain or lose points from players who are part of the much bigger "island" and their ratings will eventually adjust. This is what happened when the players outside the U.S. came to play events like Worlds and the USDGC in the first years the ratings system started a decade ago. They played in the large island of the U.S. and their ratings gradually got adjusted. These ratings were brought back to Europe, Japan and other countries and helped adjust the ratings of those smaller disc golf islands relative to the U.S.
So it's true that leagues provide the opportunity to improve your rating only relative to the other players on your island. If you want to improve it relative to bigger or other islands, you need to also play in those other locations and player pools.
Rater of the tossed arc.