Implausible. Dunn's lexicon and grammar were originally published in 1978 and 1979, respectively, or not until several years after the first iteration of the Tékumel setting (1975). Still, someone died and made apostrophes and x's de rigueur for fantasy languages. Seems to me the cliché was already in place by the pulp era.

(Incidentally, <x> is simply common phonetic notation for a velar or uvular fricative, what English speakers to tend to represent with <kh> or, in words of Celtic origin, <ch> or <gh>. And the apostrophe is a widespread convention for glottalisation, of which there's a lot of Tsimshian.)