There is no evidence
that what we call parables found in scripture
are fictional.
Not every story with Jesus in it is a parable. But he did sometimes employ the use of parable, according to scripture.
When Jesus spoke to the Roman centurian and healed his servant - this was a real event and you can tell that by the lead in and every thing surrounding the event. Very real. Very true.
However, in the parable of the talents you can see by the lead in that it's meant to illustrate a moral, and as such there may never have been a real man who gave one of his servants a coin who subsequently buried it.
now, if you believe there was a real man who had servants and etc and this was all a literal story, that's fine too.
Matthew 13:35 tells us Jesus’ parables were speaking of things of old, and it is referring to Psalms 78:1-4, which doesn’t sound like He’s making things up, but fulfilling prophecy by passing along stories from ancestors.
I'm not trying to say anything against your belief. Certainly, if you believe that in the parable of the talents, for instance, that the servant who buried his coin was a real person then that is fine. It doesnt hurt anything to believe that.
But since the meaning of parable is a fictional allegory it does seem strange to argue that it's not fiction at all and that somewhere a servant buried his coin.
The "of old" portion of that verse you quoted, is meaning or speaking of the hidden truths that were from the foundation of the world.
Those hidden truths that were true from the foundation of the world is talking about the underlying meanings of the stories, not that a servant who buried his coin in the dirt was true from the foundation of the world, but the meaning of the story is a truth from the foundation of the world - such as if you, as a servant of Christ bear no fruit after your salvation, then your going to be thrown out of the Kingdom of God..
These truths from the foundation of the world are revealed to us in the Messianic age. Praise God. But whether or not there were three literal servants or whether it was a story He made up to illustrate a point doesn't matter or doesn't affect the faith of most of us.
If it affects your faith then honestly it doesn't matter. There could be three literal servants, I certainly can't say that I know.
But you should know that when it comes to the parables, the underlying meanings are what is absolutely necessary to your faith understand, because they are the truths from the foundation of the world that is imperative for us to know.