Replacing quick-cooking oatmeal in a two-ingredient cookie recipe with cooked steel-cut oats - why doesn't it work?
If you want to use steel-cut oats/oatmeal, you'll probably want to start with a recipe that calls for it. You can substitute old fashioned oats instead of quick-cook oatmeal in most (possibly all) cookie recipes but you can't substitute cooked oatmeal without making major adjustments. 

In this case, old fashioned oats are specifically called out in the recipe as a substitute with the note that quick cook oats will give the best result:


  
  Quick oats give the best results, however you CAN use rolled oats. Use Gluten Free Oats for gluten free cookies.
  


Oats are a dry ingredient. There's no water in them so they absorb moisture from the other ingredients to cook. In this case, the banana. When you cook the oats first, you're introducing a lot of extra water and it's likely to make them take much longer to cook and change the texture.

In a recipe for cookies made from instant steel cut oats, I found this substitution note:


  If you’re using traditional steel cut oats (not quick cooking), you’ll want to cook them first, just like when you’re making oatmeal. To use cooked oats, simply substitute one cup of cooked oatmeal for the instant raw oats listed in the recipe, and add an extra half cup of flour (or more as needed to achieve a non-runny dough consistency).


You might be able to salvage this recipe by doing something similar but if you really want to use steel cut oats, you'll get better results by finding a recipe that actually calls for them. If you want to use this specific recipe, use old fashioned or quick-cooking oats.