The Myers-Briggs assessment merely says that we’re predisposed to behave in certain ways, not that our behavior is limited to one direction or the other. According to the theory, we use both preferences of any dimension, but we’re innately predisposed toward one.
A right-handed person prefers their right hand. The fact that they’re capable of using their left as well, and may even have become very proficient at it, doesn’t render their designation as “right-handed” less accurate. Likewise, the fact that someone prefers Introversion doesn’t preclude them performing in an Extraversion capacity — it simply means that it will require more of their energy.
Furthermore, the Myers-Briggs assessment actually does have a means for determining the degree
to which a person identifies with a certain preference. It is called the “Preference Clarity Index (PCI),” which measures how clear an individual is about a particular preference
— slight, moderate, clear, and very clear.