Should I Ground a Generator Powering an UPS?
What they're referring to here is a ground rod

Generally speaking, AC mains electricity in the US needs to be earthed (grounded), with the neutral and protective earth (ground) wires bonded to each other and to a grounding electrode system at the main service entrance, to keep the mains from rattling off to some high voltage relative to other grounded things (such as pipes and metalwork, or you and me) and putting excessive stresses on insulation as a result.

However, if you are simply setting a generator out and firing it up to plug a few things in out in the boonies, driving 8' of ground rod whenever and wherever you want to use your generator is considered impractical, so the NEC offers us an "out" in the form of 250.34(A):


  (A) Portable Generators. The frame of a portable generator
  shall not be required to be connected to a grounding electrode
  as defined in 250.52 for a system supplied by the generator
  under the following conditions:
  
  (1) The generator supplies only equipment mounted on the
  generator, cord-and-plug-connected equipment through
  receptacles mounted on the generator, or both, and
  
  (2) The normally non–current-carrying metal parts of equipment and the equipment grounding conductor terminals
  of the receptacles are connected to the generator frame.


Given that the UPS is cord-and-plug connected to the generator, and you in turn have things plugged into the UPS, I would consider the UPS no different than a portable power tap (power strip) for the purposes of this section, and the equipment plugged into it supplied indirectly (via the UPS) from receptacles mounted on the generator.  (The reason why the section is written as it is is to distinguish this case from a case where the generator is supplying building wiring via an inlet.)

However, I would run a ground wire back to the intersystem bonding point anyway (which moots this debate)

The problem with the logic above starts when you add the cable modem to the picture.  Cable modems invariably are grounded; however, this grounding is not a connection to mains ground, but to the cable plant ground.  Normally, the ground block at the CATV entry point is connected via a wire to the grounding electrode conductor or intersystem bonding termination block, and this means that the two grounds are really one and the same.

However, with your generator isolated from the building wiring, electricity that gets onto the cable TV system from your generator can't get back to the generator to trip a breaker (or even a GFCI, under extreme conditions), and this is a serious problem due to the shock hazard it poses.  So, I would run a wire (6AWG bare copper will do) from the grounding terminal on the generator to the intersystem bonding termination block on your house, provided you have one that is.  If you don't, you can run it back to the grounding electrode conductor instead and use a lay-in tap lug to attach is bonding wire to the grounding electrode conductor.  This avoids violating the spirit of NEC 820.100(D)/830.100(D), as this bond wire provides a fault path back to the generator for faults from the generator onto the CATV plant.