You are wrong. The backstop is only replicating things that are already present when you have EU membership. Since it’s already included in EU membership, no special agreement is required. It’s only when you try to reproduce these things for a non-EU member that you need an agreement like the backstop.
People above have given an example of how countries within the Schengen area can unilaterally introduce border controls with other Schengen area countries e.g. from Germany into Denmark.
https://ec.europa.eu/home-affairs/w...sas/schengen/reintroduction-border-control_en
The EU positively requires there to be border controls between EU countries where one is within the Schengen area and the other is not.e.g. Gibraltar / Spain border. The EU does not require open borders between EU nations as evidenced above, indeed in some situations it requires border restrictions with identity checks.
THE EU is permissive of an open border between Ireland and NI it is not directive. Either the UK or Ireland could introduce border controls and not be in breach of EU law.
The backstop in requiring bilateral agreement ie giving the EU a veto on the UK introducing border controls is requiring a higher level of 'openness' than is required under the Schengen arrangement. Pointing this out does not mean I am arguing for Brexit or against an open border; I just believe that the Backstop is not the best solution to the issue at present.