Not sure I buy it entirely. I've found that once you become an expert in a particular vendors technology to the point you have a reasonably complete mental mode

Not sure I buy it entirely. I've found that once you become an expert in a particular vendors technology to the point you have a reasonably complete mental model on how it works and how to solve all kinds of problems with it and the specific solutions tailored to specifc problems, then learning another vendors specific technology comes at deep, deep, discount. Tbh if you are an expert in X and I need you for Y and Y is analogous to X and there is time allowed for ramp up, then it's all the same to me.