The polygraph result constitutes the opinion of the polygraph examiner based on the procedure that he or she has performed. (Similar to the diagnosis of a doctor based on his examination.)
The opinion does not necessarily constitute proof. Proof, as required by a court of law, has to satisfy certain criteria.
A mere opinion does not meet these criteria. The polygraph result was never intended to provide proof.
The adage applies: To know is not to prove! It provides a very solid indication to assist with the continued investigation.