Sir Keir is savvy enough to know that he is never going to obtain the position of leader while telling everyone what everyone already knows, that Corbyn was a disaster, and the party lives in fairy land.
That doesn't win friends at the top table, which he needs to secure the top slot at that same top table.
If he is successful, you will see him drop that softly softly approach and actually start doing something to wake up the party, move it forward a hundred years, and start acting like a party that people may one day think has a reasonable shot at forming a government.
But not until he is successful.