Maybe, knowing exactly where these are would help.
Mount Apad Runan is even higher and appears to be a relatively soggy and humid place today. Given where most Nepenthes grow today I do understand why he might think it was to increase moisture availability.
Snake pelvises and whale femurs. Not everything we see today is an adaptation to present needs.
Not much elevation is needed for temperatures to drop precipitously. While the area is safe from frost today the higher elevation areas get comparatively cool during the winter nights. Succulents generally do poorly in cold places. Has this species been around for a while and may this be an adaptation to escape frost during the ice ages? It wouldnt take much soil to protect a plant and its roots, and in this case water filled pitchers, in a marginal environment.
If it works well enough then there would be no pressure to switch back to the more standard form.
Borneo................