Nixon talked with the South Vietnamese generals in 1968, urging them to obstruct President Johnson's peace negotiations. Both parties expected progress towards peace would help the prospects of Nixon's Democratic opponent.

Nixon even discussed this with the bankers that backed him.

Nixon's actions may have been a violation of the Logan Act, so having the FBI investigate was proper. Even supposing they were not illegal, they were treacherous in spirit.

Ultimately, Johnson decided to keep the secret reports that proved Nixon's treachery, so as to avoid stimulating distrust of the US government when Nixon was at its head. That was well meant, though we would today consider it naive and foolish.

Johnson also had one of his officials take those secret documents away rather than leaving them in the White House. The article says nothing about the motivation for this, but perhaps it was to make sure that Nixon's treachery would be revealed some day — decades in the future. He may have been afraid Nixon would destroy them.