Jesus tells his opponents that if he alone bears witness about himself, his testimony would not satisfy the ordinary standard they are invoking. He then points to other witnesses.
The language belongs to the legal and social world in which important claims were established through corroboration.
The Torah repeatedly requires two or more witnesses in serious judicial matters, especially Deuteronomy 19:15. Jewish and wider ancient legal culture therefore attached importance to corroborated testimony. John's Gospel repeatedly uses courtroom-like language of witness, testimony, judgment, and truth.
Jesus invokes John the Baptist, his works, the Father who sent him, and the Scriptures. The point is cumulative testimony, not uncertainty about his own truthfulness.
The legal background gives structure to what can otherwise sound like a loose list of proofs. Jesus presents a case in categories his hearers understand.
John's larger narrative likewise asks readers to evaluate witnesses to Jesus' identity. The Gospel's claim does not rest on a solitary unsupported assertion but on converging testimony culminating in Jesus' works and the Father's witness.