Which cake are you referring to? There are a bunch on that page, and I don't see lettering on the Sweet Sea Anemone cake...am I missing it? (Haven't had my coffee yet...
My mistake. The rainbow cake at the bottom.
Gotcha. )I did something similar on my animal print sweet 16, Yankees Graduation cake and the #1 on the jungle cake) I used modeling chocolate, which basically functions just like fondant (just not a fondant fan, myself!) You roll a long skinny 'snake' (as my kids would call it!) and just shape each letter, I usually taper the ends to a point and make it curl in (or out) into a spiral. They should firm up if you let them harden/stiffen for a day or so (however long it takes fondant to set up).
Good luck! HTH!!