From the comments, I think I'm missing something.
I do not think any of us disputes that the NEC sometimes requires grounding electrodes for generators.
The reason the sticker caught my eye was that it specified 25 ohms or less.
If that label is part of the generators listing and labeling than 110.3(B) becomes a factor.
It may be imposable to achieve less than 25 ohms to ground.
Picture your generator at the circus, what do they do, install one or two ground rods?
Have you ever tested them to see if they had 25 ohms or less?
And then we get into the need for 25 ohms at all.
Why 25, what will that do for us.
The generators we installed where portable generators entirely cord and plug connected.
The generators themselves where not bonded as they where tied into bonded switch gear.
In this case the generators frame was grounded by the EGC we ran from the switchgear with the feeders.