Shemeska
Adventurer
Actually, if the Dragon magazine entry on the Mercykillers is anything to go by, there will still be this aspect of Sigil -- Arwyl Swan's Son was mentioned as specifically from Toril.
It's not exactly pure canon, but it puts the idea of "alternate Worlds" in 4e, so there is precedence.
I noticed that as well. I'm not sure if Rob managed to sneak that one past the editors or if they didn't mind. But beyond that mention, the article was well written and touched upon all the sources on the MK's, including the mentions in Dragon during 3e (Nilesia's rumored fate and all).
It did deviate from the core 4e default cosmology in having that setting mixing element, so it'll be interesting to see if WotC's design team themselves keep with that in the future. If they don't, something of Sigil's flavor would be lost.