Agree, judging anyone by any characteristic is detrimental, especially when it comes to learning.
The key is to always know your audience and design your learning accordingly.
You may want to read the work of Ned Herrmann who spent many years researching how the brain worked. He developed the Herrmann Brain Dominance Instrument which is a tool that measures thinking *preferences*. Using the Left and right brain side of the brain isn’t about personality, it’s about how we prefer to think. And you are correct in saying that everyone uses both sides of the brain. And the majority of people have a *preference* for one side or the other. This is reflected in their communication, decision making, problem solving etc.
So I would challenge that this is a myth.