Roots only grow due to cell expansion (obviously cell division occurs in the root meristem, but the visible root size increase we see is due to cell expansion after new cells are divided). Cell expansion only occurs due to increasing the water content in the cell, like filling up a chain of water balloons the root grows. The water content in the root cells can only increase if there is water in the root environment for the roots to uptake. So basically what I am trying to break down for you, is that the concept of letting your media dry out to 'make the roots hunt for the water and grow deeper' is completely idiotic and based entirely off of trying to humanize a plant... but plants aren't human, and don't chase water if they are in a dry media because their roots literally can't grow when its too dry.
Overwatering does not mean there is too much water for the roots to handle, it just means you are watering so often or so much that your media does not have a chance to drain to a point where micro pockets of oxygen can exist. Roots not only need water, but they also need oxygen just like us to respire and grow. Coco coir can hold so much water and is also well drained enough that it is very difficult to over water, but still possible if you are keeping the coco soaked 24/7.
Both of these problems, over or under watering, will cause root growth stunting that will likely translate to slowed growth of your plant over all. However, the important thing that GL mentioned, is that there are so many other variables that can also cause root growth stunting, including genetics, salt levels, nutrient levels, disease, pests, environment, you name it. The more you grow, and the better notes you take, the easier it will become to spot when there is a true problem or when things are just following their normal course.
-Meeks