(Note: I am going to mention disciplinary canons and also the Scriptural canon; rather than constantly differentiating like that I'll just call the former 'canons' and the latter 'Scripture')
Apparently, this must have been an accepted canon in his day. I know the Syriac Christians have a similar canon as well if I'm not mistaken?
Indeed. For St. John the most recent kinda-sorta official pronouncement on the Scripture would have been at the Council in Trullo, a supplement to the 5th and 6th Ecumenical Councils, though the Orthodox see it as a continuation and part of the 6th Ecumenical. This meeting happened in St. John's lifetime--when he was about a teen. In the 2nd canon the people assembled in Trullo listed all the earlier canons from various sources (councils, fathers, etc.) that were accepted and binding. In doing so they accepted canons from earlier times that did not match with other canons regarding what was in and not in Scripture. Specifically they accepted:
1) Apostolic Canons - Excludes all the deuterocanonicals except 1-3 Maccabees; lists Sirach as a readable/teaching book; the NT given has the 27 familiar books, but in addition to that "two epistles of Clement; and the Constitutions."
2) Council of Laodicea - Includes Baruch and the Letter of Jeremiah, but otherwise has the 'Jewish' Scripture list for the OT; has a 26 book NT because of excluding Revelation.
3) Council of Carthage - Gives the same OT as the one accepted by Catholics at Trent (with those seven particular deuterocanonicals); has the familiar 27 book NT.
4) St. Athanasius of Alexandria - He included Baruch and the Letter of Jeremiah in Scripture, but excluded Esther; he hold to the 27 book NT; he considers Wisdom of Solomon, Sirach, Esther, Judith and Tobit as readable/teaching books, in the same category as the Didache and the Shepherd of Hermas; there is no mention of Maccabees or others.
5) St. Gregory the Theologian - He has mostly the same Scripture as the 'Jewish' one, excluding all the most-often-discussed deuterocanonicals and others, though as with St. Athanasius he also excludes Esther; he excludes Revelation from his New Testament.
6) St. Amphilochius of Iconium - has the same OT Scripture as St. Gregory, though adds that "some approve the inclusion of Esther"; he accepts the four gospels and most of the letters traditionally attributed to St. Paul, and he also accepts Hebrews but notes that "some call that to the Hebrews spurious"; as for the rest of the NT: "Of the Catholic epistles some say seven, others only three must be accepted: one of James, one of Peter, one of John, otherwise three of John, and with them two of Peter, and also Jude's, the seventh. The Apocalypse of John, again, some approve, but most will call it spurious." Many people (including most mentioned in this post) tried to finish off their listing of Scripture with an assurance that they
were giving you the real deal list and that you should ignore the others, but I especially like that of St. Amphilocius, who after mentioning multiple different disagreements and uncertainties, says: "This would be the most unerring canon of the divinely inspired scriptures."
This does not outline what must be excluded, or for that matter what must be included, in Scripture; rather, it approves various Scriptures that had been used at various times and places.