I think this is only partly true. Freerolls are indeed good for beginners: they help players get used to tournament structure, learn how to play under blind pressure, and gain experience without any financial risk. For newcomers, they are one of the safest ways to enter poker.
However, freerolls also have downsides:
the skill level is often chaotic;
many players go all-in with almost any cards;
because of huge fields, consistent earnings are usually very small;
the playing style in freerolls doesn’t always translate well to paid tournaments.
So freerolls work well as practice, for building discipline, and as a chance to build a bankroll from scratch. But if the goal is serious improvement and long-term profit, at some point you still need to move into regular buy-in tournaments.