I swear it WAS going to be HL:EP3. Since Gabe explicitly said it wasn't EP3, he sort of did not leave the whole thing open ended any more. Even a "maybe" would make everyone know for sure it'd be about HL:EP3.
So, instead of Yes/No, appear like a liar or not and all.. they just canned the whole thing, last option to do without having to part with anything on HL:EP3.
Edit: Or Gabe realized announcing/teasing L4D3 would not be such a smart thing to do, considering millions of fans with pitchforks wanting EP3 instead of yet another Left 4 Dead.