There's a lot of other factors to consider... exercise and type of exercise, macronutrient breakdown, how large the surplus is, etc.
edit: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5786199/ have a look at this, it shows that hypercaloric diets will produce weight gain, and macronutrients have an effect on composition.
Again, lot of other factors at play here, have a look at the linked paper.
Also, regardless of what the body's primary storage mechanism is, it's just obviously true that adult mammals experience changes in the amount of protein stored on their bodies on the same timescales as weight gain and loss occurs. Anyone who has ever started or stopped an exercise regime has experienced this.
The study is evidence that macronutrient composition has an effect on how much fat you gain, and I gave some reasoning for why this is the case. If you're not convinced then I suggest you do some more digging, I don't have time to find the all relevant papers for you.
> it's just obviously true that adult mammals experience changes in the amount of protein stored on their bodies on the same timescales as weight gain and loss occurs.
The body has both catabolic and anabolic processes running concurrently all the time, you'll see increase in one kind of tissue when overall one of these processes is more active than the other. caloric intake and macronutrient breakdown obviously affect these processes, as does exercise.
This is an honest question. I've never understood this.