The terminology used is confusing. People often say that if you went on Medicare on February 1st, then you were not allowed to contribute in February or later.
But the way the calculations are done (and dmertz is right that TurboTax is doing it right), in fact, the calculations don't ask for when the contributions were made.
So if your annual HSA contribution limit is $675 (as in your case), the system does not ask in which months these contributions were actually made. If you contributed nothing in January but $675 in February, then this would be acceptable, since the excess calculation is made on an annual basis.
When they say, you can't contribute in February and beyond, they are assuming that you contributed the full amount allowed each month already. Thus, if you contributed $675 each month, then as soon as you had Medicare, you would have to stop contributing. But really, the excess is calculated on an annual basis.
P.S. your HSA administrator should not be calculating your excess contributions amount, because they do not have access to your tax history and situation. TurboTax has access to your complete tax situation and therefore is in the position to accurately calculate your annual HSA contribution limit, which is calculated on form 8889.
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