The military has always frowned upon the spreading of conspiracy theories (they were called "rumors" in my day). It was well known that membership, or advancing the cause of certain organizations, would have a severe impact on one's military career.

Attendance at a "Stop the Steal" event, unless a crime was committed or the person attended in uniform, or unless specifically ordered not to attend, should not be a problem. If a person is denied a security clearance for attending a political event, he may have a course of action in the courts.

Onward and upward,
airforce