UPDATE - I found what I was looking for. As I suspected, EGM 56 (March 1994) was NOT the first published mention of Nimbus Terrafaux.
I have discovered he was mentioned before this, in the "Most Wanted Trick" section of EGM 53 (December 1993), page 148. It's only a small bit, very easily missed. Here is a scan of the article:
http://imageshack.com/a/img197/6545/6ip9.jpg
And here is the full text:
ELECTRONIC GAMING MONTHLY - ISSUE 53 (DECEMBER 1993) - PAGE 148
TRICKS OF THE TRADE - CHEAT SHEET - MOST WANTED TRICK
Mortal Kombat keeps coming back as the most wanted trick! People just can't get enough of this game and all of it's hidden attributes. This time, the Genesis version comes into the picture with another juicy rumor about a hidden character! It has been said that there is a kickboxer named Nimbus Terrafaux hidden in the Genesis version of the game. Here are the details. Supposedly, you are to wait for Reptile to come onto the screen and say, "Look to La Luna." At this point, you are to put in some sort of code and press START before he leaves the screen. The game restarts and you should be able to select this new character. Rumor also says that his face goes across the moon every so often, but you can't get to him with the DULLARD code. Do you have any ideas? If you have the method to get to him, or have a good idea, send it in! We'd love to print it! [PICTURE] Could this be the infamous Nimbus Terrafaux?!
As far as I can tell this is the first published information about Nimbus Terrafaux. Obviously we now know the head flying across the moon that they refer to is actually that of Fergus McGovern.
This find also finally disproves the popular misconception that Nimbus Terrafaux was an "April Fools" joke by EGM. The rumor was a hoax, yes, but nothing to do with April Fools. The character was first mentioned in the December 1993 issue, and mentioned again in the March 1994 issue. And if you understand how cover dates work, they are the date when the issue is taken OFF the shelves, so it was obviously never intended to be an "April Fools" joke. And to put a final nail in the "April Fools" mis-information, in EGM issue 60 (July 1994), page 18, they state that their only April Fools joke for 1994 was mispelling the word "Streets" as "Streeets" on the cover of issue 57 (April 1994). Which is a pretty weak joke, but the point is their Nimbus hoax was nothing to do with April Fools.
I suspect you are correct ded that the information on the wiki is wrong, and that Boon never did an interview hinting about Nimbus.
Hope people enjoy this obscure trivia! Nimbus for MK10!