I was doing meal prep both before and after, so I can attest to the fact that I didn’t change my diet down to the g amount of skyr I was eating at lunch. So no, I didn’t subconsciously change my diet.

Assuming a 150lb person it takes an hour of moderate swimming to burn 400 calories.

I mean obviously I wasn’t doing moderate swimming, I was fighting for the top places in swim.com at the pools I was swimming in, and the number is low, and my weight is different but let’s use it and show my point. Assuming 3500 kcal per lb of fat and ignoring all other details.

If you do 1h a week for half a year you are potentially at about 3 Lb or 2% body mass loss.

If you however you do 2h each weekday you are close to a potential loss of 30 Lb or 20% of weight.

Now obviously you could argue that the first case won’t work to lose weight and that you’ll just subconsciously eat a bit more to offset it, and the outcome isn’t even that large to begin with and would disappear in fluctuations of your weight unless you are consistent about hydration levels and food intake prior to stepping on the scale. But in the second case you will definitely end up changing your weight even without changes to your diet.

That's just the energy used for the exercise itself, it doesn't count the extra calories needed for your cells to repair themselves. Powering the machinery of repair takes energy and raw materials. Those extra raw materials (protein, fats, and carbohydrates) are taken from normal diet leaving the body in more of a deficit, which feeding behavior usually corrects for by increasing intake