You're right, Pax. He was from the 9th century, not the 10th. If he existed in the 10th century, he'd have probably seen Knez Vladimir Dukljanski (Jovan Vladimir). Besides, I found out that on the Geneaology of the rulers of Montenegro at montenegrina.net, Archont Petar is listed to have ruled during the 9th century, opening up a possibility that Archont Petar is indeed Petar Gojniković.