They most likely hard-coded the "correct" checksum value in the game.
Knowing a lot of Genesis games did that, and it was also common to find "Master Codes" to allow Game Genie to get around that, I just google "Mutant League Football Game Genie codes" and find
1. R19T-86VA - Master cod, must be entered
Use a Genie Genie decoder to find out the corresponding ROM address/value and hack that.