I don't think that is necessarily true. Data about a real existing person could have been collected either in (now lost) written or oral form, and could have survived the decades between the death of Jesus and the composition of Mark.
Sure. As I have said several times before, the gospels and Paul’s letters
“could” have contained reliable evidence that the authors had obtained from informants who actually did witness things. But the reason I put it the way I did, was to emphasise that in fact none of those gospels or letters do produce any such evidence quoted from or otherwise reliably shown to have come from any other known individual who did actually have the evidence of Jesus first hand.
So all we are left with in any of the biblical writing, is evidence of the authors beliefs about other unknown people believing that yet other people called disciples had once known the evidence with their own eyes. And what they knew was that there was once a messiah who was supernatural and completely impossible. That is not credible or reliable evidence from any of those religious writers who, as I say, could have no evidence of their own, and who give no indication of anyone else who actually knew the evidence and told it to them either.
We are told that Jesus had "brothers"; if not blood brothers, at least close associates, surviving into the 50s and 60s. Collections of sayings of wise men can be made and retained, even by word of mouth, especially if they are recorded in a poetic form. I can still recite poems that I learned many decades ago as a child, and I can teach them to my own grandchild.
If they were not actually blood family bothers, then they were not really "brothers" at all, were they!
Apart from which, nobody who was supposed to have been a brother of Jesus, ever wrote to confirm that he was indeed a family brother of Jesus.
Instead, all we have (yet again) is later religious authors who wrote to say that other unknown people had once talked of brothers of the Lord, whatever that might have meant. But that cannot possibly be reliable or credible evidence of any of the authors who wrote such things, actually knowing if any of that was true in any sense at all.
Finally, some of the data in the gospels are derived quite evidently from OT "prophecies" but others are not, or not clearly so. The crucifixion of a messiah being one example. The Christians really had to exert themselves to find anything in the OT that could conceivably be advanced as a "prophecy" of this event. It is improbable that they started with these lame "prophecies" and falsified the event on that basis.
Well first of all you do not need to find every word about Jesus in the OT. It is more than sufficient if you find more than a couple of such incidences, to conclude that this is not some mere coincidence and that in fact those gospel writers were using the OT as the source for their messiah beliefs. Paul's letters for example repeatedly stress that the OT was certainly the source for his Jesus beliefs.
But iirc, in fact the crucifixion story was very likely obtained by Paul from what Paul thought was OT prophecy, and we have been through that before several times. You don't need to find a word-for-word prediction saying Jesus will be crucified by Pilate. What seems to have happened is that Paul, and later the gospel writers, were searching in an OT of messiah prophecies which they all regarded as the most important thing that governed their every waking moment, for any passages which might have conceivably seemed to them to be refereeing in any obscure or "hidden" prophetic way to the coming of Gods promised messiah.
So what was happening, was apparently that individuals like Paul, Mark and Mathew, would find certain passages, or if not actually having written copies then they would come to believe from verbal accounts that certain passages said various things, whereby they could imagine that the passage, whatever it actually said and whoever or whatever it was actually originally referring to, could be interpreted by them (Paul, Mark, Mathew) as a coded or "hidden" meaning about the messiah who Paul came to believe was the Jesus figure named by Moses c.1000BC.
That might include not merely taking one complete sentence from one particular book of the OT, but actually taking bits of various different sentences from different books of the OT, and piecing the parts together to form what seemed to people like Paul, Mark, Mathew, to be the
"revelation" of God’s true meaning in the coded messiah messages
"hidden so long" in the OT.