I add a bit of nuance to this. Allow me to take your statement and restate it this way.
If history is employed, it ought not to be counter-factual according to accepted understandings of the time.
And even this is still fails to capture more that needs to be said. There are two ways in which current understandings (“history”) could be used:
in an incidental or illustrative way --i.e. to help develop understanding of something else entirely.
in a foundational way – i.e. the teaching builds on the mentioned history in a way that depends on its actual truth like a conclusion which depends on a premise.
Here is an example of the first one above. I am an ancient teacher trying to help a pupil understand how something (an animal perhaps) circles its prey, and I say “It does for a small time just like the sun circles the earth.” Such a teaching remains intact even if it is discovered later that the sun doesn’t circle the earth at all, because I wasn’t teaching that it does so; and a contemporary reader of my words would be 100% in error to think my point was about anything other than the animal. I was employing apparent motion of the sun to make another point entirely --and the latter point is the only real one.
Jesus’ parable about the mustard seed (or any of his other parables) are also in this category. It is wrong for us to think that the prodigal must have historicity before we can accept the truth of its teaching. We should look for the actual points he is making to us rather than try to make something of the incidental understandings of the time that he employs for illustrative purposes. Now, yes – it ought to at least be an accepted understanding of the audience, lest it be a poorly chosen illustration. One supposes they all knew of such dangers along roads connecting cities like Jerusalem or Jericho so that the Good Samaritan story connects with them, but you and I are 100% wrong if we make such things into necessary premises of his parables or think them in any way about those incidentals. The mustard seed is an even better illustration of my point in this regard since we now know it indeed is not the smallest of all seeds. But Jesus teaching that employs this understanding is not one whit touched or troubled. Only the modern notions of what inerrancy ought to look like become troubled and must commence with their rescue gymnastics.
The #2 (foundational) way more matches your expectation. If I was teaching my pupil, “the sun is moving very fast because, you see, it circles the earth once every day” – now here your assertion that my “history” ought to be true does apply full force. I have made it foundational (not incidental or merely illustrative) to what I am teaching. When it turns out that the sun doesn’t actually move around the earth, my teaching fails. (Yes --it turns out to be right anyway, but because of galactic motion – not for the reason that I gave.) One rightly holds the conclusion suspect when a false premise is discovered.
I think our real issue is that many want to turn incidental, illustrative things in some passages into full-fledged premises. What they are protecting is not so much Scripture itself (as they insist) as their own modern notion of what infallibility or inerrancy should look like. In doing so, the real aim of the passage (the discernment of which ought always to be our aim) is obscured or even jeopardized and violence is done to Scriptures.
Edit: more than a few clarifiying edits were added to improve wording and more closely make my points.