Jamie's right, developing multiple projects at once is standard operating procedure in Hollywood. It would be bad business to focus everything on just one project-- if that project doesn't come off or is a flop you're in real trouble. You have a better chance of surviving a failed project if there's another (or twelve) in the wings.
There are always Muppet movies in development-- it's been that way for years upon years. We've heard bits about some of the possible projects (Muppet Haunted Movie ring any bells?). But there is a huge difference between in development and pre-production. The Princess Bride was in development for about a decade, at various studios, before Rob Reiner brought it to the screen.
I suspect the bio is basically telling us two things-- 1) Henson wants to continue making Muppet feature films and 2) Lisa will be the Henson in charge of developing those films (maybe not producing them, but developing them). Strikes me as good, even if it doesn't mean something is going to happen soon.