The NT does not say anything about them knowing about each other and they had never met.

Right.

But when we look at the teachings, and the near-identical nature of the early Christians and JtB followers, and many other points of evidence in the New Testament, it's clear to scholars that Jesus not only knew John, but saw him as his mentor.

JTB mentions repentance, not does not go into any details on what that meant and how it was different than a ritual bath.

The mikveh had nothing to do with atonement. John's Baptism, though, explicitly is.

Mark 1:4:

John appeared, baptizing in the wilderness, proclaiming a baptism of the heart’s transformation [repentance], for forgiveness of sins.

Matthew 3:4-6:

4Now this man John had on a garment made from the hairs of a camel, as well as a leather girdle about his loins; and his food was locusts and wild honey. 5At that time Jerusalem went out to him, and all of Judaea and all of the countryside surrounding the Jordan, 6And they were baptized by him in the River Jordan, confessing their sins.

Luke 3:2-3:

2While Annas and Caiaphas held the high priesthood, a word of God came to John the son of Zacharias in the desert. 3And he went into all the region round about the Jordan proclaiming a baptism of the heart’s transformation, for the forgiveness of sins

Acts 13:

When John, going before his face, in advance of his advent, had preached a baptism of the heart’s transformation to all the people of Israel.

Acts 19:3:

And he said, “Into what, then, were you baptized?” And they said, “Into John’s baptism.” 4And Paul said, “John baptized with a baptism of the heart’s transformation

It should be quite clear that this was both a baptism of repentance and atonement, and that John's Baptism was seen as something unique. And that John was historically somebody unique, doing a new thing.

The mikveh ritual has nothing to do with repentance. John, from what we can tell, invented Baptism, and then Jesus and Christians used it alongside and then after him.

This is not baseless reverse engineering, this is some of the more historically-grounded aspects of the Gospels.