Quickbreads vs Cakes: Understanding Mixing Ingredients -
Your premise for Type 1 is correct, however you seem to have misunderstood Type 2: It is no problem to continue mixing the butter-sugar-egg mixture, but as soon as the flour is added, the gluten starts to develop and you should just mix until combined.

For Type 2, creaming the butter and sugar helps to incorporate a lot of air into the mixture, resulting in a light and fluffy texture. This continues when the eggs are added. Proper creaming takes some time, but at some point further mixing does not improve the result any more.