no, great barrier reef was given as an example of a natural wonder taking up more than one hex, it wasn't said to be the only one taking up more than one tile, and you can CLEARLY see that everest takes up 5+ hexes, the entire 5+ hexed size garganteoun in one mountain with a snowy cap that covers the middle of the "one mountain" but in terms of tiles the snow is in the corner of 5+ hexes, it is 5+ hexes big, look at it again David, it is clear to see.
Heres a slightly more zoomed out view, you can clearly see it is 5+ enitre hexes.
The safe assumption would be that each hex will give +2 prod and +3 gold, that is just an assumption, but it makes more sense than only 1 hex out of the 5+ it covers being productive.
I mean its clear to see it's on a mountain range, and it goes out 1 hex further to the east and a few hexes south, but the main 5 hex cluster with the snow ontop definitely are all the Mt. Everest if not the entire mountain range.
edit:// now I have looked a little closer, I think the snow is actually only in 3 hexes, with 2 normal mountains above Mt. Everest, 1 East and several south. If the entire range isn't considered "part" of Mt. Everest then It may be just the three hexes in the middle with snow that give the bonus, and you can clearly see the 3 hex wide Mt.Everest Snowy Peaks three little peaks infact that are slightly higher than the normal like mountains.
Darn its really hard to tell... the snow might even be considered to be in 4 hexes including the south mountain, so it could be 4 hexes wide. Suffice to say, it is not simply "1 hex big"
That really is an impressive wonder.
Edit part2:// looking closer, why the hell is that scout standing on a mountain... maybe that isn't considered to be "part of Mt. Everest" if thats the case... then we are back to 3 or 2 hexes. OMG SO CONFUSING, we will have to wait for an ingame look at the monstrousity.