This is so cool that you were able to have it genetically tested.
I get random seedlings all the time that come in via other trees. You never told us the back story about how you obtained the seed. Did it come from an adult tree, did you pick it up off the ground, did it come from a nursery? What is the origin story?
Here's the full origin story. I have a strangely detailed record of the history of this tree.
On June 3, 2019, I was driving along a dirt road in Northeast Kansas (Nemaha County) along a creek, when I spotted huge drifting mounds of cottonwood seeds along the side of the road. I had come prepared with ziploc bags, since that was actually on my grandfather's farm, and I was looking for souvenir cottonwood seeds. I scooped up two bags worth of seeds.
When I got back to the Bay Area a few days later, I planted the tufts of cottonwood seeds in a long container with potting soil as the main substrate, but with a surface dressing of red lava rocks. A week later, I had probably a dozen cottonwood seedlings.
On June 9th, I went out of town for a business trip.
By the time I got back on June 12th, a slug had eaten every single cottonwood seedling! I caught him in the act. None were left whatsoever.
But my hopes were restored again, when a week after that, this seedling appeared:
Naturally, I thought it was another cottonwood. I protected it from slugs, and treated it as my last hope of having a cottonwood from my grandpa's farm.
But in no time at all, it became clear that it was not a cottonwood at all. By mid-July, it was clear it was some kind of elm.
So I assumed it was the notoriously weedy Siberian elm (which is all over the place in Kansas).
HOWEVER, by September, the tree, now already over 4 feet tall, had mysteriously large leaves. Far too large for a Siberian elm. And yet, it really didn't seem like an American. I then thought it was a hybrid of some sort.
And I've been trying to figure out what it is ever since.