At that point Sanders and everyone else who wanted a $15 and hour minimum wage increase would have been involved in intense behind the scenes negotiations within the Democratic caucus with the likes of Sinema and Manchin. None of us can say what the end game would have looked like in that scenario, and the degree to which the public posture taken by Sanders, Manchin and others were part of initial bargaining positions.
The Senate parliamentarian ruled against including a minimum wage increase in the Covid relief bill and Democrats lacked the votes in the Senate to over rule her. Even Biden was signaling that the game was up for including a minimum wage increase into the relief bill at that point. Bernie not initially agreeing to $11 an hour with Manchin, in discussions that preceded the actual debate on amendments, might have built pressure for both of them signing off on $12.50 as a compromise in the final hours if any increase had been possible. However all of that is moot. NO raise was possible in that bill, not to $9, not to $11, not to $15.
If you really want to blame someone for blocking inclusion of any size minimum wage increase in the Covid Relief Bill that just passed through reconciliation, it is more logical to blame our President than Bernie for that. Biden always signaled that he would accept the ruling of the Senate parliamentarian and seemingly made no effort to lobby Democratic Senators to over rule her decision. Biden holds more sway over Democratic Senators, especially moderate ones, than Sanders ever will. To be clear, I do not in any way blame Biden, he always knew that there would not be 50 Democratic votes in the Senate to over rule the Senate parliamentarian.