Rival NPCs absolutely have extra hidden stats. Look at these numbers and tell me they don't.
I've got 100 more stamina than them.
You might claim it's guts difference. The difference between 300 and 350 Guts is 1.47x stamina use in late phase vs 1.44x, and if the +400 invisible stats from career also affects guts, the difference is even smaller. 1.31x vs 1.3x.

You can't convince me that the NPCs aren't cheating.