One cool thing about this is that it will allow Nintendo to absorb NLG's proprietary tech, and Luigi's Mansion 3 shows they have one hell of a Switch engine.

Sounds great, what games are they responsible for?

Luigi's Mansion 2 and 3, Punch Out Wii, and Mario Strikers are their most well known efforts.

Nice grab then. Fond memories of Strikers 

Nintendo would still have to pay their creditors. It wasn't a massive amount of money but it was still quite a lot for not that much gain. 

At the same time, they probably weren't going to keep the Mario and Luigi games going since they were doing poorly after Dream Team. It would make more sense to absorb the employees and put them working on Mario projects (as they are familiar with the IP). 

I do agree. My point was just that sometimes it can be worth to buy a company in debt, you can pay almost nothing to owners and just have to pay debts (and sure that will always depend on how much debt and potential profit in the future there is).

These are the guys that make Punch-Out & Luigi's Mansion right?
So they already primarily made nintendo games.

(also the ones behinde metroid federation force)

Yep from what was said here those were the 2 main IPs they worked on, and seems like they have only made games for Nintendo for like the past 10 years.

For Nintendo they made Luigi's Mansion 2 & 3, Punch Out Wii, Federation Force, (though I more blame Nintendo for that one) and Mario Strikers 1 & 2 for Gamecube and Wii respectively.

They've also made some licensed multiplatform games like NHL Hitz Pro, Spiderman Friend or Foe, and Captain America Super Soldier.

For the last several years they had made only Nintendo games, but now that they own them Nintendo can bring them under their first party umbrella in order to work more closely with them and more effectively share resources, technology, and personnel, as well as ensuring they'll never leave.

Nintendo will expand them as they have with Monolith.