In any political system, the first duty of the leader to maintain the clear legitimacy of his or her power. If that legitimacy derives from the military, then that support had better be clear, for example.
In our constitutional system, therefore, the first and most critical task of the President is to protect the legitimacy of the election. That duty is greater than protecting your own win. This legitimacy is seriously threatened by any credible evidence of external interference, particularly by a traditional adversary. For there to be such a serious threat there does not need to be conclusive proof of such interference, or its impact. A CIA finding obviously rises far beyond such a standard.
Therefore, any president-elect who ignores, trivializes, or refuses to take seriously such a finding has to be in gross violation of his as yet unsworn oath.
While everybody, including the Electoral College Electors, has to consider how to respond to this simple and unarguable fact, we need to be honest and direct that it exists.
To put it a slightly different way, Nixon’s White House dismissed the significance of a “third-rate burglary attempt.” They did not deny that a burglary had occurred. They did not, as least nominally, fail to cooperate with the investigations.