The PI warranty disclaimer has absolutely nothing whatsoever to do with a neutral-ground bond or the lack of one.
I believe most all portable generators under 5 kW these days (whether open frame or not, or inverter or not) do not bond the ground and neutral conductors as OSHA rules permit their use in this configuration without GFCIs. Perhaps this is only applicable to 120V generators rather than 120/240V split phase generators; I don't feel like trying to parse through the OSHA requirements. Many generators this small would probably not be 120/240V split phase anyhow.