It makes perfect sense. I can't understand why you would care about perfect grammar in a comic strip, especially since it's supposed to imitate spoken language.
Oh, and cats qualify as "furniture" even if they're alive, by the way.
If the incorrect grammar were part of the joke that would be one thing - but it's not, it's just incorrect grammar. The statement makes no sense if you say "in" rather then "into".
As one blogger put it a while back, there are really two states of a cat: "alive" and "furniture." He postulated that Schrodinger's Cat was in the "furniture" state. About as useful as a dead horse, except for the Godfather references.