The NEC does not define diversity. In fact, I'm almost certain that word does not appear in the NEC.

Do the treadmills plug into receptacles?

To me, you are dealing with Receptacle loads in other than dwelling units which is covered by 220.44. You might be able to convince the AHJ that table 220.44 applies, which is first 10 kVA at 100% demand factor, remaining kVA at 50%. But he/she might decide that 220.50 applies since treadmills presumably have at least one motor and must be calculated in accordance with 430.24, 25 and 26. So 430.24 or 25 unless the AHJ gives permission to use 430.26.

Using 220.44 and assuming 30 treadmills at 100% FLA (16A):

30 x 120 x 16 = 57,600 kVA

First 10 kVA + 0.5 x 47,600 = 33,800 kVA

That's 281 Amps for the receptacle loads.

Not sure how much you can shave from that trying to apply 430.26.

I don't know if you ever been to Orange Theory, but around here that place is PACKED. Every single unit can be used during a popular class and some time of that session can potentially be spent at 100%.

I would advise two things. First get in touch with the AHJ and talk it through with them. Make your case and see what they say. If they insist on a particular method, your hands might be tied. Second, see if you can somehow get your hands on historical data as the information note in 430.26 suggest. Does the owner have other location with the same setup? If so, what is the max demand according to the utility?