Playing another key, say G on a D whistle, is part of the learning curve. Buying a new whistle only to use it for its basic key really defies the purpose. I don't think pointing out the common alternative interferes with your explanation, it complements it.We are trying to explain the different key whistles, & why you would buy them
It is common practice to use a D whistle to play in D and G, Am (dorian if you like) and Em (dorian again) and A is within, relatively easy, reach as well. Works perfectly fine.Diatonic instruments are not the best for playing all keys, for that you are far better off with a chromatic instrument.
This
doesn't make it easier to play in all keys. Even a keyed whistle or (simple system) flute isn't really a fully chromatic instrument. Keys will help with the accidentals but you don't quite step into playing in any key at all ( though some will be easier to fall into than others).