Good one! The calculation was fine, the data wasn’t.
The OP wrote:
It’s called BMX, where one buy the SPX index and then writes ATM call options.
I grabbed Yahoo data for BMX.
The index is actually BXM (ticker ^BXM)
So one more time using the index data from Yahoo, the stats below are from 1993-11-13 to 2022-07-06
BXM_timed SPY
Annualized Return 0.0941000 0.0965000
Annualized Std Dev 0.1644000 0.1904000
Annualized Sharpe (Rf=0%) 0.5723000 0.5068000
maxDrawdown 0.4579059 0.5518945