I don't recall him saying anything like that. Nothing close. My recollection is that he was complimentary about Johnny and seemed open for a reunion (and biographers have noted he held out hopes for at least a year or two). He never suggested that his best work was only possible with Marr.
Johnny has done lots of good records since The Smiths. He's made some mediocre records, too, many more than Morrissey has. But you make it about Johnny's ego when in fact Johnny (as he states in the full interview to which Maurice has provided a link) has always had modest ambitions. His one goal in life seems to be to play his guitar with people he likes. Period. As he says in the interview, he started off merely wishing to play on a 45, not take over the world, and he's been doing the same thing since he was 14 or 15. Just a guy playing in a band, which is what he is. I don't think The Cribs or Modest Mouse hold a candle to The Smiths, but his work with them seems entirely consistent with his personal philosophy. I believe him when he says he feels no need to go back to a partnership with Morrissey (and I believe Morrissey when he says the same about Marr).
This is true in only a limited sense. Johnny left The Smiths, but as many people over the years have attested, Johnny felt pushed out of The Smiths for various reasons. Now and then when he walks onstage with Modest Mouse or The Cribs he probably can't help but think, "Damn, I wish I was playing with Morrissey, Mike and Andy". Then he probably remembers that he'd be playing with Morrissey, Mike and Andy after three hours of dealing with attorneys, record company execs, caterers, van drivers, and making sure one of the roadies got his paycheck.
Morrissey and Johnny have remained totally consistent over the years. The former is a true pop star, the latter is a musician who likes making music for himself and his friends. I hope they continue to remain true to themselves and refuse each and every offer to reform. Johnny's re-mastering efforts to remove all the studio shit added to The Smiths' songs in the 90s means a million times more than a reunion tour ever could. When your band is accepted as standing on the same hallowed ground as The Beatles, the story's over. There's nowhere left to go.
Except to the bank, of course. And nobody wants to see them do a cash-grab.