The big spike in testosterone is responsible for changing the physiology of the body. Same way a woman grows body hair, facial hair, an Adam's apple, a deep voice and a more masculine jawline if she uses large amounts of exogenous testosterone. However, less testosterone once fully developed does not equal lesser amounts of facial hair or heightening of the pitch of ones voice. You're oversimplifying things. It's not a myth, it's a largely recognised fact by every health organisation and physiologists the world over. This sort of thing is literally why males have testosterone.
Oh, so you were talking about how whole food is somehow better than a shake and how we must east certain foods to boost our testosterone for gains, and now you're going to tell me to stop the broscience
The thing is, it's not even broscience - it's actual basic physiology. Your example was a bad one as it's not quite ideal to be getting half your calories from carbohyrdrates on a maintenance or fat loss diet - I did say that macronutrient balance does play a part. However, you could get a large chunk of your carbohydrates from cola on a fat loss diet and still lose weight if your caloric intake is in check. It's a carbohydrate - carbohydrates are converted to glucose to use for the body's various needs. All carbs cause insulin spikes, whether it's sugar or oats. Sure, higher GI carbs will raise insulin quicker, but it'll then drop quicker too. So, bigger picture considered, 40g of carbs whether it's from sugar or complex carbs will produce the same effect in terms of function. The magnitude of the insulin spike is the same, sure the insulin doesn't spike as high but it's a more gradual spike and lowering with a lower GI carb, meaning longer spent in a state of heightened insulin. Only downside would be that sugar isn't as satiating.
Again, I did point out that the balance of macronutrients does play a part. However, calories are the main thing that will determine what happens in terms of fat loss or gain. You're not going to lose fat on a keto diet if you're eating at maintenance or above - it will simply aid in fat loss during a deficit by keeping glycogen stores depleted and using ketones.