It is correct. As long as your star multiplier is above 1, you get matched by MMR and not rank. It says so explicitly in the Ranked FAQ:
Q: How does matchmaking work for each season?
A: There are two matchmaking pools: matchmaking by rating (aka. MMR) and by rank.
When your star bonus multiplier is bigger than 1, or you are in Legend rank, you are (generally) in the first matchmaking pool; when your star bonus drops to 1, you are in the second pool.