The sole source for this information are notes taken by the FBI, according to the article you linked, with the recording that allegedly backs up these claims as

The sole source for this information are notes taken by the FBI, according to the article you linked, with the recording that allegedly backs up these claims as sealed until 2027. I'm in alignment with the American studies professor quoted in the article who says: "My experience has been that there are some transcripts of FBI surveillance but no actual tapes released so we are dependent on their reporting and whether we can trust it since its purpose was to gather negative intelligence on MLK. I did not know about the rape incident that King is alleged to have witnessed without intervening."