That's simply not the case.
Each gospel writer was trying to do the same thing -- for his church (which at that time meant a small group meeting in each other's homes) to collect the accepted stories about Jesus into a narrative, and by default leave the other stories out. They're writing the canon for their group.
And each gospel writer had to deal with all of the different stories that were circulating orally and in codeces (a codex was a kind of book or pamphlet, as opposed to a scroll) at the time and place that he wrote.
In fact, that was the whole reason for writing -- to sort out the mess. Keep in mind, we have literally thousands of Christian codeces from the first couple of centuries CE, and that's undoubtedly a tiny fraction of what was in circulation.
Mark was the earliest biographer we know of. And obviously, it was the first successful attempt to write a canonical biography of Jesus.
Matthew and Luke -- written after the fall of the Second Temple and the destruction of the Priesthood, probably a few years before the start of the 2nd century -- used Mark because it had become accepted by their groups (among others). It wasn't the only source, by any means, but it was a privileged source.
I'm also surprised by the glib way that Price tosses out phrases like "some additional post-resurrection narratives". Here, he's talking about the apostolic witness, which sets up the whole dividing line between the true-apostolic (Jamesean) groups and the Pauline/gentile groups, as well as the ascention, which places Jesus firmly in the company of the highest of the annointed men of God, such as Elijah.
What seems to have happened here is that Price understood the basic source scholarship, but then made incorrect assumptions about the paucity/plethora of sources to draw his conclusion.
Then there's this:
Let's be perfectly clear about one thing. This "equally valid explanation" is made up out of thin air.
Nobody has ever run across any "expanded" Mark or any reference to, or even hint of, any such thing.
"Expanded Mark" is a pure invention of the imagination.
What's worse, Mark is a relatively crude, and certainly early, document. It has been added to. But no reason is known -- or, to my knowledge, has ever been proposed -- why anyone would want to cut material from Mark.
The claim of equal validity here is simply false.
As for his comments on John, it's definitely gentile, and it certainly asserts the miracles, but it then consistently makes the point that seeing the signs isn't the important thing, it's faith in Jesus (and having your whole household join you, if possible).
There's really nothing remarkable there.