Strictly speaking, both can be true; the term "market based" simply accentuates the positive, i.e., the extent to which the plan respects free-market principles, rather than the extent to which it violates those principles.
A Massachusetts insurance commission sets minimum rates and minimum coverage for the insurance providers (not the insurance companies, the rules come from the state).
Then, everyone must sign up or they will charge you through your taxes.
If it were market based, there would be no rate setting, the insurance providers would compete for your business, and the deals would be so good you’d sign up.
Health insurance rates in MA are going up between 8 and 12% for the next fiscal year.