Weird ... any research I can find indicates that this *is* an issue at high/extreme altitudes (airplane, radio towers on mountaintops, etc.), but nothing talks about inverters in use at 5000' to 10000'. It's apparently an issue for "medical" and/or "industrial/military" equipment. Not much mention of consumer equipment. For example:
"Creepage and clearance" are keywords for the susceptible equipment's electronics.
None of my equipment has nose-dived because of continuous operation at this elevation. No electronics have failed, outside of their normal warranty period, in over a decade at this elevation. Magnum LF inverter is outside of its 5-year warranty, and still chugging ...
Can't tell, but this wording (if in Schneider docs) joins the dozens of other warnings that the lawyers stick in there, and that might get a vendor out of warranty (it's highest purpose?) for consumer equipment.