It’s not like hot swappable one. The keyboards is to adjust the positions of the keys and add/remove keys freely. You can change Planck layout, Ergo layout, number of keys freely (1st picture) and can change the board too (2nd picture). I think Moonlander has enough keys. But after practicing it, maybe people would recognize some keys might be not used. They want to use more compact keyboard. A keyboard like DK6 enables this use case changing the board to smaller one, and removing some unused keys.
I have never seen this type of keyboard before the keyboard DK6. Recently I was told by my colleague who ordered this one. I am not sure how it looks like.