Total myth... Just think, if the cage aggression theory were true, what would breeders with a hundred ball pythons do? They all feed their balls in the enclosure, or do you expect them to move each one to a tub/feeding container? That would take hours to feed them all. So does feeding in the enclosure lead to aggression? If it did, all breeders would have 100 aggressive ball pythons. LOL It just doesn't happen.
There's also a lot of good reasons to not try to move them during feeding, other than that too. It actually increases your risk of being bitten, since you're trying to move the snake around while it's in feeding mode. And then moving it back to the enclosure right after it eats causes stress, and can potentially make it regurgitate the food it just ate. Also, moving the snake to begin with causes some level of stress and can make it snap out of feeding mode- thus causing it not to eat at all. Just nothing good about it.
And for personal experience, I feed all my snakes in their enclosure.They are all docile and chill as can be.