Aatube
The report's argument is that making more people eligible for the existing means-testing while still making the rich pay (those making 400% of the US poverty level currently ride the subway as much as those 0–150%!) has been calculated to be one-third the cost of free busses. The program isn't adding or restricting means-testing, which is what gradually happened to ACA Medicaid as you mentioned, but relaxing means-testing that already exists.
My argument against the report is that free fares would increase the bus ridership especially for the poor, invalidating the reasoning that it "wouldn’t reduce the costs of travel for most riders, since 76% of daily transit trips are on the subway".
fwiw the accounting savings are already factored into the $900M price tag