It was that, after 65, you're supposed to be on Medicare. Period. And they way they enforce it is they charge you extra when you do finally get on it (presumably after your private insurance becomes just too expensive...)
I'd be delighted to be proven wrong, but we were told repeatedly, including by Medicare itself, we Shall Be Penalized.
https://www.medicare.gov/basics/costs/medicare-costs/avoid-p...
"It’s important to sign up for Medicare coverage during your Initial Enrollment Period, unless you have other coverage that’s similar in value to Medicare (like from an employer). If you don’t, you may have to pay an extra amount, called a late enrollment penalty."
The penalty is intended to stop people from saving money by forgoing insurance entirely, then hopping on when they get sick. Ask your insurer if they can provide a certificate of creditable coverage for the period. I wonder if a quick consult with a lawyer with relevant experience would be of use.
https://www.medicare.gov/basics/costs/medicare-costs/avoid-p...