There are so many mixtures of meat and other ingredients used and ‘overworking’ is not a well defined term, so you can say whatever you want about overworking the meat mixture and what that results in and it will be true for someone.
My meatloaf recipe as best I can put it is this:
2 lbs. ground meat
approx. 2 cups bread in various forms
mix with stuff in the fridge and in the cabinets
smear the top with some kind of sauce
bake 45-60 minutes at 350F depending on the shape and size of the loaf
wait at least 10 minutes for all the delicious and healthy grease to re-absorb into the loaf
consume as much as possible while still hot
consume the rest, if any, on sandwiches another time
Follow the recipe exactly and you will be pleased with the results.