How could Mohammad have met any Christians other than Catholics since the Catholics claim that there have been no other Christians since Christ? They claim that until after the Reformation there were no others who would have considered themselves Christian.
You have to remember that the definition of a Catholic changes depending upon what is needed to defend Rome.
Often we have been chastened for using "Roman Catholic" or abbreviations because there is only one Catholic faith, while here i purposely simply said "Catholic" and that results in a protest as being too inclusive.
Whether you accept the Latin notion of catholicity (what most English speakers mean when they casually use the word “Catholic” and read it back through Church history) or the Orthodox notion (that we are the One Holy Catholic and Apostolic Church and catholicity is sobornosty, not just universality), there were plenty of Christians in Mohammed’s time who were not in communion with either Rome or Constantinople (which at that time were still in communion, the Popes of Rome not yet having deviated from the Holy Orthodox Faith by accepting the heresy of the dual procession of the Holy Spirit): there were residual Arian and gnostic heretics (for example the Lombards were still Arians), the Assyrian “Church of the East” had broken communion with the rest of the Church by rejecting the condemnation of Nestorius in 431, the various Monophysite churches (the Copts, Ethiopians, and Syrian Jacobites) were out of communion since their rejection of the Council of Chalcedon (451), and the Armenians since their embrace of the Henoticon of the Emperor Zeno.
No, the Latins don’t claim to have been the only Christians until the Reformation: they vainly accuse us Orthodox of being schismatics and are recognize all groups I just catalogues (whom the Latins and we Orthodox agree are heretical, but Christian).
Incidentally, Arab Christians I know uniformly hold the tradition that Mohammed was a missionary from the Assyrian Church who went rogue — a position supported by the fact some sections of the Qu’ran are nonsense as Arabic and perfectly good East Syriac, the liturgical language of the Assyrian Church to this day and in those days the main language in what is now Iraq.