Biden's campaign and the party gave the Sanders campaign every reasonable consideration and accommodation. Negotiations and compromises were made in good faith. That part is OVER now. Biden doesn't need any former rivals "advising" him how to run his campaign.
This type of backhanded and back-channel "leaked" critique serves no good purpose. It's just something that creates resentment and distrust. It's pulling scabs off of old wounds and pouring salt in. It's a source of negativity. Negativity generates apathy. Apathy discourages voter turnout. Low voter turnout gives Republicans a chance to steal the elections.
Surely people have better and more productive things to do with his time than to take cheap shots at Biden from the sidelines. The GOP is already doing enough of that. Why add to it?