It kind of depends on which Jesus.

Based on your disclosure, you are concerned with the cannonical depiction of Jesus. In which case what he was allegedly discussing with these two seeds parables were the growth of faith.

But as can be seen from 2 Cor 11, before the gospels are written there are other versions of Jesus floating around, and a number of the topics Paul writes about to Corinth in 1 Cor overlap with the group who thought Jesus was effectively citing Lucretius in his seed parables. You can even see that in 1 Cor 15 Paul refers to sown seeds in the context of the human body where he also talks about a first physical man vs a second spiritual man. It's only in 2 Cor that we see Paul talk about sown seeds relating to proselytizing.

This other extra-canonical group with ideas overlapping with early Corinth and their "other version of Jesus" had a very different eschatology from cannonical Christianity, and in their case they explained these parables as effectively relating to Greek atomism, but with the specific language of Lucretius. In fact, the apocryphal text they were following is filled with sayings of a Jesus very concerned with addressing the philosophical points of Leucretius in a rebuttal of the Epicurean belief there was nothing after death.

Essentially, there were at least two versions of Jesus in antiquity. One that was talking about evolution and matter being made of indivisible parts, and one that was not saying anything about this. The one that was canonized and a third of the world believes in today was the latter one, but personally I find the former a bit more astonishing coming from an age where these ideas were fringe and unconformable concepts but is now being read in an age where they are largely recognized as facts.

It actually reminds me of the structure of the third seed parable. That in the earlier age when they couldn't tell what ideas were wheat and which ideas were weeds it would have been better to have waited preserving both until such a time it was clear which ideas were wheat and which were weeds. But instead the church perhaps prematurely labeled this other tradition as weeds, it was banned on penalty of death to even possess, and we only know of it today from works buried and lost for millennia and only recently rediscovered.

(This too was an idea in Lucretius - that it would be a mistake to throw out explanations for things before knowing for sure what was correct.)

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.