(c) The optional method is only allowed when there's an HVAC load.
OK, I got this part wrong, my apologies, I should have read the section more carefully, and I'd like to revise my comments. My new understanding:
The optional method is allowed for single family dwellings without any such heating/cooling load restriction. For two family dwellings, it's allowed with effectively a 50% penalty (which is more or less the effect of calculating it for 3 identical units instead of 2) (and which seems weird to me).
So now I understand the issue to be that the demand factors for 3 or more dwelling units for the optional method are only allowed when each dwelling unit has electric cooking and also electric heating or cooling. While the standard method does the demand factors on a per appliance type basis, so you'd still get to, for example, use a demand factor for electric cooking when there's no electric heating or cooling.
Further, it's seems that the NEC text doesn't allow this phantom HVAC load approach. Because the requirement on electric cooking, 220.84(A)(2), has an exception allowing the demand factors with a phantom 8kw cooking load. But the requirement on electric heating or cooling, 220.84(A)(3) does not have an exception.
Nonetheless, it makes absolutely no sense not to allow the phantom heating or cooling method. You could design a building with each unit having a small one ton mini-split inverter heat pump drawing 3 kW, supplemental to a central plant HVAC system, and it would satisfy 220.84(A)(3). It makes no sense to say "remove the heat pumps, and now your service size goes up".
Cheers, Wayne