I'm confused by the use case for this. The keyboard gets a cable running to a monitor. Might need a power cable as well but let's assume usbc covers both.

An alternative is a raspberry pi on the vesa mount, or attached to the monitor arm. The cable to a keyboard is now optional, wireless USB being much easier than wireless displayport.

Keyboard can now be flat too.

When is this a good idea?

The marketing blurb that's linked makes it quite clear that this targets retro hobbyists, who want a modern take on the C64. It's not really meant to be a practical design.

It still is a more practical design than a flat keyboard, which only masochists would use willingly.

So if they had made one change, it would be fantastic as a throw-in-the-backpack computer.

That change would be to support display port alt-mode on a USB-C port, rather than only having mini-HDMI. If they'd done that, you could plug AR glasses like the XReal One straight into it, and not need a separate screen. Your entire compute becomes a keyboard+power, glasses, and wireless mouse. That would be really nice: two cables, total, one for power to the pi and one from the pi to the glasses.

As it is, you need an hdmi to usb-c converter, which also needs to be powered, another couple of cables, and more of a setup faff each time. It sounds minor, but it's a missed opportunity. For me it turns it from "take my money" to "eh... I can do better."

I thought these kinds of ~affordable computers in keyboards were obviously aimed at families/young users plugging into an existing TV in the living room, like a modern take on the C64.