It just matters that you pass the bill. If “all-50 states” preclearance is what puts Manchin behind a bill that he thinks ought to pass the filibuster, then that’s half the battle right there, because when it doesn’t pass, he can point at it as the reason why he gives up on the filibuster.
It also makes Republicans take a stand on a very clear issue.
Pretty good idea, really. Equal treatment under the law is good.