Yes. But the answer is a bit more complicated than that.

Social Security law says that it is illegal to sell private health insurance (a Marketplace plan in this case) to someone who is entitled to Medicare. Entitled in this case means actually enrolled in Medicare, not just eligible for it.

For most people who turn 65, they are entitled to Medicare; they paid into the system for 40 quarters and can receive premium-free Part A. Some people however, even though they are 65, are not entitled to the benefits, either through immigration status, not having paid into the system enough, or not having lived in the US long enough. These people are able to enroll in a Marketplace plan and receive subsidies.

To clarify, the ability to enroll in Medicare is irrelevant. It is the cost of Medicare that determines the Marketplace subsidy. If you need to pay for Part A, you are entitled to a subsidy. If your Part A is premium free, you will not receive a subsidy from the Marketplace.