I wouldn't doubt it was that one, and the reporters scrambled their info. If Frank Oz was currently working on a Muppet Movie Script, we'd know it. If we can hear faint rumblings of someone producing a script, we'd hear of Frank's script loud and clear.
The fact of the matter is that, even though they had the script Jason was working on, they announced Cheapest at a D23 (which the only thing they correctly predicted was a Hallmark ornament and a line of Ugly Doll type toys that didn't last long). Now, there's nothing that said that they were actually reworking that script (if one even exists)... and even if they were, they'd use the current Muppet team to punch it up, not someone who's in another state of their career. Among other things, Cheapest would have been a terrible way to relaunch the Muppets... it's too experimental a concept. It would fly after TMM, GMC, and MTM... not so much as a launching pad project.
The Muppets WAS the film that needed to be made, better or worse. Now that it did as well as it did... impressive only due to the fact the box office was weak the following weeks after the film and that several other children's films with higher budgets were released in a week and a half long period (some of those movies didn't even make worldwide what The Muppets made nationally in the first week), they can make any film they want.