When watching the fastest run on any% co-op I noticed that they do some weird grey cloud thing so the bosses (boss 2 and 3) take way more dmg. Is this some known bug because I don't see the top score in any% using this bug.

We inject a magic drug into our brain cells and this allows us to go past our humanly limitations. In other words - we become gods.

Step 1: Supernova right in front of them.
Step 2: There is no step 2!

Looked it up in the instruction manual. When you play co-op you get an ability bar that goes up just like in single player mode but to activate it in co-op both players have to press L3 or R3 at the same time and then you get a dmg boost or a shield. I thought it was first a bug in the game because no one else did it but it was only available in co-op mode. So if there are any other co-op players out there now you know because I for sure had no clue.

Muckerman would we be able to submit co op ILs

I don't think so, there is no category for that in ILs. I have only done co-op on any%.