The jar they found had two marks of kashrut – it was sealed and thus presumably free from defilement, and it had the high priest’s stamp to indicate that the highest religious authority of the generation vouched for it.
There remains a problem, however. If the jar was sealed, why did it need the extra feature of the high priest’s stamp?
There may be an analogy to current events in the kosher food trade, whereby some manufacturers and/or restaurateurs assure the kosher consumer that their kashrut is reliable even without a certificate from the rabbinate. The response to this current claim must be that the manufacturer or restaurateur is not necessarily objective or unbiased, and to have the stamp of an outside authority indicates objectivity.
There is a further very important element – food technology is so complex that questions of kashrut need to be decided by experts. However good the manufacturer or restaurateur may be at their job, they cannot be presumed to be qualified to make halachic judgments.
But let’s get back to the jar of oil and the high priest’s stamp. The high priest was not just anybody, nor just an ordinary priest. Because of his extra degree of anointment, he was (or should have been) the one on whom rested the God-given responsibility of knowing the Divine will and understanding the Jewish soul of the people to whom he ministered.
Some high priests betrayed God’s trust, but in theory at least the high priest of a particular generation was regarded as sufficiently imbued with spiritual conscience that he would never have dared to certify anything which would have besmirched the soul of a Jew.