Use of NPT Thread Cable Gland in Ex "E" Enclosure?

One possibility is that the port for the Ex e enclosure already has a metric thread and an NPT thread on the gland won't fit it.

That's what I understood from the text, other posibilities are that the gland connector won't fit or it will be way too loose to be tightened.

It will depend on the type of enclosure. If it is something like a junction box, you most likely don´t have enough material thickness (if it is an Ex"e" JB) to provide for an NPT thread, in this case some sort of parallel thhread with a washer and locknut would be used.

If it´s something like a transmitter housing, then it is just a matter of specification during ordering. All manufacturers that I know of offer the choice of either metric or NPT threads.

The type of gland has a bearing on the matter too. Ex d glands are marked to that effect, and designed to have a minimum of, say, 5 turns of taper thread engaged in the Ex d fitting before going tight, which is a function of the thickness of the Ex d enclosure. Ex d equipment is built like a Russian tank.

Ex e enclosures can be much thinner, in which case a parallel thread and a back-nut is appropriate; one doesn't get parallel thread in NPT though one does at, say, M20, and stuffing glands for Ex e enclosures with a back-nut can be widely used and are readily available from all sorts of suppliers. There is no such thing as an Ex e gland.