I knew there was literally a Krispy Kreme next door to my gym for a reason

It's not the donut or the brand. It's simple carbs that can be processed faster. I bet drinking chocolate milk would yield even better results.

My psot workout shake is

200ml semi-skilled milk
scoop of dry oats
banana (sometimes frozen berries too)
scoop of PB
1-2 scoops of whey

I like to get a good mix in terms of nutrients

600 calories none of which coming from protein, seems less than ideal

donuts actually have whey in them, so that’s probably it

Donuts like that are still only 3g or so protein.

If you're working out hard enough, you will burn through the calories so almost any food is fine. Donuts are fine but I looked at the nutrition info and it says 11g of fat so probably not optimal. What needs to be kept in mind is that with studies, the participants are usually untrained and have lots of noob gains in the tank. For people like that, the extra calories has somewhere to go so to speak. For someone trained, it might not partition so well.

A better case study is someone like monster0ultra who is trained. 6 months ago, I was eating whatever I wanted and lots of ice cream post workout, often a pint of Haagen Daaz on top of the PWO meal. I was around 189 lbs then. Currently, I eat clean and try to keep the fats low and carbs high and my extra calories come in the form a porridge of rice, raisins, and almond milk. I'm not fully clean right now though because I'm not too strict when it's social eating which happens semi frequently. I'm currently around 183 lbs.

I had/have abs in both situations so the ice cream and junk still "works" but the 6 pound difference would suggest that cleaner works better.