This chap may say this but on what basis should we regard him (or any Christian apologist) as being authoritative?
Don't be silly: in general conversation about politics if you said 'Boris' to most people in the UK they would know who you were referring to - but using a shortened version like this would make his lies no more or less believable than if you used his full moniker. Apparently 'Boris' refers to himself by another of his forenames when dealing with family and friends - so what?
If you mention just the forenames of made-up characters many people will be able to identify them and go on to describe them as having certain attributes even though they are wholly fictitious - try it with 'Harry' or 'Sherlock'.
Boris isn't a common name, neither is Sherlock. So neither have to be disambiguated by adding their other name. I had to think which Harry you meant, because I don't read or watch Harry Potter.
The four gospel narrators initially disambiguate Jesus' name, eg Matthew 1:1, Mk 1:9, Lk 1:32, Jn 1:45.
From then on, the narrators call him just, Jesus, or 'he', since the reader knows who he's talking about. However, in quoted speech, his name is qualified. Jesus was a common name, so the other characters would have had to qualify it when talking about him or to him.
I think the point is that someone writing a long time later in a neighbouring country, would not necessarily have known that 'Jesus' was such a common name, and therefore would not have been so thorough in the disambiguation of his name in quoted speech.
The same with Simon, which is also disambiguated because it was a popular name. Simon Peter, Simon the Leper, Simon the Tanner, etc.
If the narrators did write their gospels while outside Israel and some time later, we can infer that they would not have written this way unless they were recording what was actually said.