How did he adhere to any enemy of the United States? This is usually described as physically joining with another nation to carry out an act against the US.Seems pretty clear to me that it rises to the level of criminal activity under the plain language of the constitution.
Article III, Section 3, Clause 1:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the testimony of two Witnesses to the same overt Act, or on Confession in open Court.
As to giving them aid and comfort, again you may as well lock up the entire diplomatic corps if you want to stretch Milley's actions to fit that definition.