Because "closets" are not on the list of places where 210.70(C) applies. [FWIW, you're never going to actually enter the space marked "F9" in the OP. That install calls for a water heater designed to be serviced from the front, and you'd be in the hallway while you work on it. But these details are merely informative; what matters is the text of 210.70(C).]
I don't mean to suggest that it needs to have two items to be a utility room. Rather, I'm saying: 3' x 3' with a 26"+ diameter water heater occupying basically all the space--that's definitely a closet. 6' x 6' with say a water softener next to the water heater, and you can walk in and close the door--that's definitely a utility room. 6' x 6' with just a water heater in it, probably that's a utility room too, just not quite willing to say that it couldn't be something else instead.
Cheers, Wayne