From DIGEST OF UNITED STATES PRACTICE IN INTERNATIONAL LAW 1975, p. 750:
The clear intent of this provision [Logan Act] is to prohibit unauthorized persons from intervening in disputes between the United States and foreign governments. Nothing in section 953 [Logan Act], however, would appear to restrict members of the Congress from engaging in discussions with foreign officials in pursuance of their legislative duties under the Constitution. In the case of Senators McGovern and Sparkman the executive branch, although it did not in any way encourage the Senators to go to Cuba , was fully informed of the nature and purpose of their visit, and had validated their passports for travel to that country. Senator McGovern’s report of his discussions with Cuban officials states: “I made it clear that I had no authority to negotiate on behalf of the United States — that I had come to listen and learn....” (Cuban Realities: May 1975, 94th Cong., 1st Sess., August 1975). Senator Sparkman’s contacts with Cuban officials were conducted on a similar basis. The specific issues raised by the Senators (e.g., the Southern Airways case; Luis Tiant’s desire to have his parents visit the United States) would, in any event, appear to fall within the second paragraph of Section 953. Accordingly, the Department does not consider the activities of Senators Sparkman and McGovern to be inconsistent with the stipulations of Section 953.[5]
He wasn't on a Congressional fact-finding mission: He was arguing on behalf of the new Administration which will be sworn in come January.