Whether or not the gospels were depicting real events, there are points where they almost certainly are depicting earlier stories of events.

For example, Peter's denying Jesus three times right around the time Jesus is having approximately three different trials to eventually convict him, in one case even going all the way back into the guarded area where the trial was taking place to deny him.

Did this happen exactly this way with the rooster and everything? Almost certainly not.

But it is very unlikely that a group of people following Peter as the person receiving the continuation of the tradition would have invented his denying the founder right when the founder was being tried, and would have made it happen a specific number of times or placed it literally inside privileged areas.

More likely is that there were stories of Peter having been denying Jesus around the time of the trials and having been spotted going back into guarded area where the trial was taking place, and then the gospels in his later tradition were trying to explain these accusations away through narrative.

i.e. Yes, he denied him, but it was only to normal people standing around and it was prophesied so it was ok.

Also, in particular in Mark you can see these splits between earlier stuff which is typically described as in public and the later stuff which is typically described as occurring in private with only a handful of people.

So with a saying like the sower parable, it was likely widely known at the time the gospel of Mark was being written which is why it was characterized as being spoken in public. But the part about him explaining it in private probably isn't even originally part of the first draft of Mark, as they jump from the shore to a private meeting and never jump back, yet are back at the shore before it moves on to the next segment. This makes more sense if a later editor inserted the explanation and the entirety of Mark 4's sayings at the shore were in public.

So did a historical Jesus actually stand at the shore talking about randomly thrown seeds? Who knows?

But regardless of that narrative detail being true, it's likely that there was a saying about thrown seeds attributed to Jesus before Mark was being written, and that either the author or a later editor was adding in a secret explanation well after the parable itself was more widely known and attributed.