Not quite, T2 to T3 is still walkable (I did it on Thursday). I didn't trust the bus so kept walking.
Just stay on the elevated walkway, turn left in T1 check-in, and take the lift down to ground floor. You then exit the airport and trundle across to T3, passing underneath Olympic House. It's a bit crunchy but actually quite quick.
Certainly true that T2 to T3 is the longest trek Man has to offer. I'd want an hour as a minimum safe margin.
The old T1>T3 through route is (I believe) now being converted to a variety of airside lounges. I assume they will re-open a more palatable through route once the current building is completed.