There is a legal doctrine of executive privilege, where the executive branch can resist subpoenas, but that privilege is limited (US v. Nixon).
Neither the doctrine of separation of powers nor the generalized need
for confidentiality of high-level communications, without more, can
sustain an absolute, unqualified Presidential privilege of immunity
from judicial process under all circumstances.
SCOTUS pointed to the kinds of cases where such privilege would be valid
Absent a claim of need to protect military, diplomatic, or sensitive
national security secrets, the confidentiality of Presidential
communications is not significantly diminished by producing material
for a criminal trial under the protected conditions of in camera
inspection, and any absolute executive privilege under Art. II of the
Constitution would plainly conflict with the function of the courts
under the Constitution.
when a claim of Presidential privilege as to materials subpoenaed for
use in a criminal trial is based, as it is here, not on the ground
that military or diplomatic secrets are implicated, but merely on the
ground of a generalized interest in confidentiality, the President's
generalized assertion of privilege must yield to the demonstrated,
specific need for evidence in a pending criminal trial and the
fundamental demands of due process of law in the fair administration
of criminal justice
Decades later, Harriet Miers claimed executive immunity (as White House counsel) to a subpoena to testify before the Committee on the Judiciary, and the district court ruled rebuffed that claim
There are powerful reasons supporting the rejection of absolute
immunity as asserted by the Executive here. If the Court held
otherwise, the presumptive presidential privilege could be transformed
into an absolute privilege and Congress's legitimate interest in
inquiry could be easily thwarted.
if the Executive's absolute immunity argument were to prevail,
Congress could be left with no recourse to obtain information that is
plainly not subject to any colorable claim of executive privilege.
However, executive privilege has been invoked dozens of times since Nixon – there are no other SCOTUS decisions.
We should set aside current political posturing. POTUS might claim executive privilege, and then the question is whether the courts would order Banon to testify. We don't know the basis for such privilege, but we can assume that it would involve national security issues (thus might be within the penumbra of the Nixon ruling). Traditionally, executive privilege has involved the person who is president, not the person who ended up becoming president, so it would be extremely surprising if the courts upheld immunity in a pre-election matter.