I think he means that Rizzo and Pepe kind of replaced Rowlf and Scooter, at least among the main six (since Rizzo was around long before the deaths of Henson and Oz, and Pepe was introduced several years later), and now Rowlf and Scooter are major characters again, while Rizzo and Pepe were barely in the last two movies (though they are still major characters in other recent things).
Quite honestly, I feel Rizzo was gaining more screen time partially because Steve was far more comfortable with him than Kermit. The only real proof of that I have is the commentary track from MFS which Kermit left at the very beginning to return at the very end with Rizzo taking most of the commentary. Also, Henson's odd decision to ease Kermit in slowly put Gonzo in Kermit's role for a few projects (MCC, MTI, MFS and painfully obvious with MT), with Rizzo giving him someone to talk to. Starting with MFS, he tag teamed with Pepe.
I really think the unique situation with the last couple of Muppet projects is the new characters used to fill in gaps left by puppeteers that died or left have replaced most new characters
with the original, classic, older ones. Brian going off to do Henson projects left us without Sal, and somehow that means no Johnny, even though Bill's still around. Even then, the only characters from MT that stuck around (thankfully) were Pepe, Johnny, Sal, and VanNeuter. So we're essentially left with just Pepe, and even he's been used more sparingly inside of the movies, yet not the promotions.
I do not think adding Walter to the cast replaces anybody. I mean, I could see Gonzo being with Fozzie in MMW, but considering he had the most to gain with Constantine in charge it wouldn't have made much sense. Scooter, maybe... The real problem? The Muppets were loads and loads of characters and while it works better in a television show, movies have always put a small core group (mostly Piggy, Kermit, and Fozzie... Gonzo to an extent) in the focus with the other characters basically sharing screen time as secondary and tertiary characters. Look how long it took to get Link Hogthrob an actual
part in movies. Maybe MTI if you count his Gong banging cameo. The only tiny complaint I'd have with the last 2 movies is that we really should have had an entire solo Electric Mayhem song in both. But considering before that, but after MTM, their biggest role was in MTI where only Zoot and Floyd had a couple lines.