John 7:15 is quoting the crowd of visitors who are hearing him for the first time. Like the rest, they are astonished. They say he is ‘learned’ without having ‘studied’ = ‘discipled’. They go on to call him demonised! Do you accept that too? If not, why accept the other at face value?
They were reacting in astonishment to someone who was, very unusually, not teaching in the manner of a disciple, which was unheard of.
John routinely used “the Jews” to refer to the leaders of the Jews, as distinct from the multitudes or the crowds of ordinary people. Cf. “for fear of the Jews,” which meant the authorities. (See the note in the NIV Study Bible at John 7:15. Scholars generally concur that Jesus did not have formal rabbinical training as did Paul.) Concerning my use of the term “courtesy,” I was saying that they used “Rabbi” as a term for one who had earned the right to be called that by his inherent qualities rather than by having formal credentials.