Thanks for pointing this out, TheRealCanadian, I have confirmed your observation that there is no totalization for Medicare. Here is an example from the US-Austrian agreement:
Although the agreement between the United States and Austria allows the Social Security Administration to count your Austrian credits to help you qualify for U.S. retirement, disability or survivor benefits, the agreement doesn’t cover Medicare benefits. As a result, we cannot count your credits in Austria to establish entitlement to free Medicare hospital insurance.
I have found similar language in the US-Canadian and and US-Belgian agreement and information elsewhere that Medicare is not totalized under any of these agreements.
However, OP I would encourage you to contact the the Social Security Administration at 1-800-772-1213 or visit your local Social Security Office to see if any reduced Medicare benefits exist that they may be eligible for if they have less than 40 quarters (as with all government programs, these are a complicated mess).