When you retire, you can (generally) opt out of whatever public retirement plan is available to you. So if they don't want Medicare Advantage, they just opt for traditional Medicare. (Ohio STRS has a medical plan - like a Medicare Advantage plan, but somewhat different).
So the only thing I can think of is that they were told it would be one of these public service retirement plans, not Medicare Advantage. And since you have to opt in to standard Medicare at 65, or later if covered by a work plan, they may have lost the option.
But - promissory estoppel isn't a good claim. Generally these benefits are never absolute. I kept my money in the STRS when I left teaching because I was promised access to medical coverage. That was an issue because I was uninsurable and it was long before either HIPPA or the ACA - and I might have been stuck without access to decent coverage. Well, about a decade after I left teaching, they changed the qualifying number of years from 5 to 15. At that point I had between 13 and 14 years. I was royally pissed, but I hadn't actually confirmed directly with STRS or with an attorney - and it turns out these things aren't actually guaranteed.
It turned out well for me, since I had the option to return to public employment and, for a 5-year investment, more than double my guaranteed retirement income. But after that I never relied on anything other than the basic retirement.
ETA: Sorted it out (probably). The issue is that NYC is paying the full premium for supplemental insurance, if that is the option retirees choose. They wanted to switch them over to a Medicare Advantage plan. They seem to have abandoned that plan - but I'm pretty sure that even if they hadn't, retirees could stay with Medicare + supplement if they pay their own premium. The issue is trying to balance the budget - same reason STRS upped the number of years for teaching retirees in Ohio to be eligible for insurance (even though we pay a portion of the premium - we don't pay all, so they changed the game mid-stream.)