As a person who has gone through different introversion/extroversion phases during my lifetime, I agree that introversion cannot be a definition of one's personality but rather a description of their current phase. I know there are some who prefer to form stronger bonds with fewer people and others who prefer to form more casual bonds with more people. However as everything else in life this depends on many complex random interactions which shape our behaviour and it is something that can be changed according to circumstances. Labeling someone as "introvert" is way too simplistic. Social withdrawal can be an effect of low self-confidence, or nourishing a need for uniqueness, or disinterest due to an illness/deficiency, or dissatisfaction with the current communication system, or being busy with other projects, etc.

Then we have the questions: Who is more introverted, a person who doesn't meet anyone in real life but chats all day long in online groups, or a person who hangs out with more people but needs privacy and alone time for a longer period of time?
Or who is more introverted, a person who works in a highly sociable environment and has learned to socialize with many customers and colleagues out of necessity, or a person who works from home and has learned to communicate with less people in a more deep and effective way? The problem is that you cannot measure just the result and ignore the parameters.