Never said he did, just that the first mention in print of a non-human Centauran species occurred in the Technical Manual.
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What page in the STTM does it describe the population of the Concordium of Planets as non-human? I'm looking at it now and I can't find it.
Couldn't say, lost my copy to basement flooding a decade ago. There's a fair amount of information over at Memory Beta, much of which references FASA Trek...
I don't like how Memory Beta (or Memory Alpha, for that matter) seems to bend over backwards to present wildly divergent stuff as cohesive. (For example, why'd it drop 'of Planets' in the name?)
Anyway the STTM SCCOM banner clearly has human mythological figures on it, and uses Greek letters, so I don't know where people are getting 'alien' from that either.