I've done no research in this, but if this were true, why are there orders of magnitude more Sanskrit manuscripts than any other ancient language?
Hot and humid climates result in (historically)rapid decay of plant and animal based writing surfaces
Were the Vedas et al memorized before being written down? Yes, of course. However, regular discoveries are made of ancient manuscripts in various Buddhist temples.
Humidity doesn’t guarantee that something will rot. It makes it more likely.
The item will need to be protected from the natural environment. Or stored in a location that is an exception to the general climate.
Compare that to the Middle East. Where the natural climate assists in preservation.