Why bother recruiting a spec ops group from a military that no country has really ever trained against

This is false, https://www.reuters.com/world/us-commandos-train-unexpected-north-koreas-shadow-2024-03-15/

https://www.nknews.org/2024/01/south-korean-military-rings-in-new-year-with-drills-against-north-korea/

whose tactics are practically unknown

Also false, https://www.npr.org/2023/06/09/1180694943/north-korea-defector-spy-kim-jong-un-us-covid-nuclear-succession

https://thediplomat.com/2021/10/unpacking-claims-of-secret-north-korean-intelligence-operations/

https://www.nytimes.com/2024/07/16/world/asia/north-korea-defector-political-execution.html

whose been specifically trained to survive and resist coups and invasions without resources or supply lines?

No evidence yet that they've been trained for this. More that these troops have been trained for infiltration (but not as applicable to Ukraine where they'd stick out like a sore thumb) and assassination (which applies to specific targets, not a general all-for-all shoot-out) as per https://www.brookings.edu/articles/what-do-north-korean-troop-deployments-to-russia-mean-for-geopolitics/

The same reason Ukraine is using drones, duct tape, and grenades.

From what I've seen on this sub, Ukraine has some high quality military grade drones and grenades, among other things. And duct tape reminds me of MacGuyver - who takes on MacGuyver and wins?

All in all, Ukraine is much better off than North Korea,

https://news.sky.com/story/north-korea-defector-tells-of-escape-and-reveals-what-life-is-really-like-in-secretive-state-12851920

And this disparity will be reflected in a display of military might against North Korean troops.