That document is referring to the surface of the building wall, not the surface of the generator housing.
I'd be interested to find out if NFPA 37 has changed. It used to be that you could locate the generator as close as 18" to the house wall though I would always want at least 36" for work access. The 60" spacing from openings in the building is the same as the older Generac publications show.
Mine passed at the 18" requirement as listed in the Generac documentation. My inspector said they default to the manufacture's recommendations since it is a fire rated enclosure.