The scholastics might have expressed his point by saying that the proper name refers per se, ‘through itself’, whereas the pronoun refers per alium, or ‘through another’, i.e. through the proper name ‘John’.
I am not sure. There is a strange passage in Mark 14:51-2 about a man who is not mentioned anywhere else in Mark, nor in the other gospels.
 And a young man followed him, with nothing but a linen cloth about his body. And they seized him,  but he left the linen cloth and ran away naked.If Vallicella is right, i.e. if the reference of the pronoun is always per alium, through another, and if the ‘another’ is always a proper name, it follows there must be some proper name that ‘he’ (intralinguistically) refers back to. But there isn’t. The man is never mentioned by name, and there is no other reference to him anywhere in the New Testament. Absolutely everything we know about him is from these two verses. The pronoun seems to inherit its reference from the indefinite description ‘a young man’. I shall discuss the reference of such indefinite noun phrases later.
What if Mark had given the man in the linen cloth a proper name, as Luke does to Zacheus in chapter 19 of his gospel?
 And behold, there was a man named Zacheus, who was the chief of the publicans: and he was rich […]  And when Jesus was come to the place, looking up, he saw him and said to him: Zacheus, make haste and come down: for this day I must abide in thy house.Yet there is still the problem that Zacheus is introduced via the indefinite description ‘a man named Zacheus’. Why shouldn’t the pronoun in ‘he was rich’ and the proper name in ‘Zacheus, make haste’ both inherit their reference from the indefinite description. What is so special about proper names?
Sometimes Mark, like the other gospel writers, introduces people without such an indefinite introduction. Mark 15:40 says “There were also women looking on from a distance, among whom were Mary Magdalene, and Mary the mother of James the younger and of Joses, and Salome.
But what is the sense of the very first occurrence of a proper name in a narrative? Does it tell us which individual its predicate is true of? Not at all. ‘Simon the leper’ is mentioned only once in Mark 14:3. ‘While he was in Bethany, reclining at the table in the home of Simon the Leper, a woman came with an alabaster jar of very expensive perfume, made of pure nard’. What information does the first and only use of the name convey about Simon? Nothing at all, beyond the fact that he was called ‘Simon’, and that he presumably suffered from leprosy. The passage could have read ‘reclining at the table in the home of a man called “Simon the Leper”‘ without loss of meaning.
In summary, in a narrative context there is nothing special about proper names that distinguishes them from pronouns. Both have to identify an individual in order for them to have their full sense, but they can only identify an individual by inheriting their reference from a previously occurring noun phrase. They cannot refer per se, only per alium. And ultimately they can do this only through indefinite noun phrases. These alone, by their very nature, are unable to inherit reference. That suggests that they refer per se, but this raises the question of whether ‘a man’, ‘a certain woman’, ‘one of the disciples’ can refer at all. I shall discuss this in the next post.
This useful page itemises all the characters, both named and anonymous, who are mentioned in Mark.