This article states it in plain English : https://www.medicareinteractive.org/get-answers/coordinating-medicare-with-other-types-of-insurance/...
If you choose to delay Medicare enrollment because you are still working and want to continue contributing to your HSA, you must also wait to collect Social Security retirement benefits. This is because most individuals who are collecting Social Security benefits when they become eligible for Medicare are automatically enrolled into Medicare Part A. You cannot decline Part A while collecting Social Security benefits. The takeaway here is that you should delay Social Security benefits and decline Part A if you wish to continue contributing funds to your HSA.
Finally, if you decide to delay enrolling in Medicare, make sure to stop contributing to your HSA at least six months before you do plan to enroll in Medicare. This is because when you enroll in Medicare Part A, you receive up to six months of retroactive coverage, not going back farther than your initial month of eligibility. If you do not stop HSA contributions at least six months before Medicare enrollment, you may incur a tax penalty.
