From the source document:
The FBI responded to Dean’s request on Oct. 23, 1969, but the part of the memo that discusses Kopechne’s travels is blacked out, leaving it a mystery whether she was in Greece or why the Nixon administration was interested in knowing it.
Jackie married Onassis in October 1968, maybe Teddy was there in August and they were curious if Mary Jo had accompanied/met him there, indicating an on-going affair ?
Follow the money, follow the booty.