Did you include Feb. 29th in your calculations? SEC requires 10-K filings 90 days after the end of Fiscal Year. That means the 10-K was due on March 30, 2024. But since March 30 was a Saturday, SEC extends the due date until Monday, April 1.
Does the 15 day period on the NT filing extension include Monday April 1? If not, then you are CORRECT.