Shot Show 2020: Where is the Grendel?

[Double Naught Spy]

So it is not a hunting round then. Why would a target round need to tumble?

Oh they sell it as a hunting round. I don't know that all states will allow it for hunting, however, because it is not an expanding bullet. They also sell their bullets for self defense.

And it isn't that a target round needs to tumble. Many bullets are designed to shoot well that will tumble on impact, even if the tumbling aspect isn't part of their design intent. This next part I borrowed from the internet...
The stability of an accurate bullet is determined primarily by its length. The longer the bullet is, the easier it loses its stability in the air or tumbles on impact with anything (brush included). You can make a bullet lighter for a given length by using lower weight metals like copper, brass or similar metal alloys.

Some/many that tumble will disintegrate during the yaw when inside a medium such an a body, creating much more devastating wounds than a simple tumbling bullet will create.

Since it cannot mushroom or fragment, "tumbling" is the only way to maximize lethality.

I am not sure 'maximize' is the right word here. Tumbling is the only way to increase potential lethality over just simply penciling through. They could have designed a bullet that did mushroom or a bullet that did fragment. Instead, they have chosen to hype a very simply design into being something extraordinary. This seems to be some clever marketing. In the video you linked, they are really trying to given the impression that the temporary wound cavity is something amazing, but the permanent would cavity is fairly unimpressive compared to expanding ammo, such as SST.
[video=youtube;nxFBUYjxnNU]https://www.youtube.com/watch?v=nxFBUYjxnNU[/video]

I have to wonder why they are going with the TUI concept instead of designing a better bullet.