The Dumarest explanation seems a little too convenient, but on the other hand Traveller is very much the Dumarest RPG more than any other single influence, so it’s possible. I’ve honestly never seen a serious discussion of the spelling before.
There doesn’t seem to be anything in Marc Miller’s background to have influenced his choice, unlike M.A.R. Barker who used a mix of British and American spellings due to, if I recall, a Canadian educator and travels in India.