The Centers for Medicare and Medicaid Services (CMS) released a fact sheet explaining why the Marketplace does not affect Medicare beneficiaries. It also distinguishes Marketplace Open Enrollment (for people who do not have any insurance) from the annual Medicare Open Enrollment (for Medicare beneficiaries to enroll in Part D drug coverage).
To read the fact sheet, click here: http://www.cms.gov/Center/Special-Topic/Open-Enrollment/Downloads/Medicare-Marketplace-FAQs.pdf