Total nonsense.
I suspect he was trying to allow for the natural slight increase in tire pressure as a tire heats up during the day due to sunlight, driving etc. However, this only makes sense if he checks the tire pressure at dawn, when it's coolest.
Also, the Caymen Islands are no hotter than much of the US in the summertime... and nobody else experiences such problems. Rims do not warp due to heat or excess pressure, just severe impacts.
IMO if he has 'warped rims', its due to the abuse that many rental cars receive at the hands of people who simply don't care about the vehicle. (They don't call them "rent-a-racers for nothing
)
As for tire pressure, I believe all cars have the manufacturers suggested tire pressure displayed somewhere on the vehicle... inside glove box, door post etc. This is a good starting point for a moderately-loaded vehicle. If it's loaded to the max, increase the tire pressure to the max shown on the tire sidewall.