Constitutionally, the admission of a new state is not actually a legislative matter, so the legislative filibuster shouldn’t apply. In recent years both Democrats and Republicans, by ending the filibuster for confirming presidential appointments (a power outlined in Article II, Section 2), have in effect agreed the filibuster shouldn’t apply to certain constitutional matters that aren’t covered by Article I of the Constitution, which lays out the design of Congress and its legislative powers. The admission of a new state is also not included in Article I. The drafters set it apart as something distinct, not a change to law but a change to the structure of government.
Admission of states is dealt with separately in Article VI, Section 3 which says, “New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress.”