BobKamman
Level 15

The source is probably IRS Pub 969, which doesn't explain (because it's written by people too young to worry about Medicare) that "enrolled in Medicare" means any part of Medicare -- not just B, but A.  And the Code section itself doesn't state "enrolled," it states "eligible" and "entitled:"

Section 223(b) (7) Medicare eligible individuals

The limitation under this subsection for any month with respect to an individual shall be zero for the first month such individual is entitled to benefits under title XVIII of the Social Security Act and for each month thereafter.