Plenty of dems have openly called to pack the courts in Congress, so Biden would have a say in court packing if there ever was a large enough margin. Biden owes people an answer on court packing, but of course he's already made it clear voters don't deserve to know his position prior to election day (how that makes sense is beyond me, I'm not even going to try to understand how he came up with that logic).
I don't see any good from Biden taking a stand one way or the other at this point, I wouldn't be surprised if he isn't sure right now other than wanting to keep it on the table. Could be a great bargaining chip a few months from now if he wins, or there could be some other reason that pushes him one way or the other.
Chuck Schumer has also floated the idea of making DC and Puerto Rico states so that the senate can get 4 more democratic senators. Not quite democratic to change the rules when you don't get your way.
Schumer can't just make DC and Puerto Rico states. He could allow a vote on the Senate floor, but PR or DC becoming a State would require a bunch of other votes as well. Quite democratic.