I feel like I'm taking crazy pills, we are directly talking about a scenario where a worker says "pay me more or I leave" and the company says "Our union doesn't want nonmembers to be paid more" or "We can't pay you more because of the union" and you're telling me "then leaving" is not bargaining.
I go to a store and I tell them I want to buy item X, but the price is too high. If they don't want to lower the price, I'll go somewhere else. They say they can't change the posted price. I leave the store. Do you call that bargaining? If you do, you have a very peculiar definition of the word.
It is not bargaining if you can't possibly get what you want.
If you are in a union shop, you can't get paid more than what's on the grid. If you want more, you need to resign and get somewhere else. But don't pretend you have a choice to do "individual bargaining".
Unions eliminate the individual bargaining power of people.
Except, of course, if we choose to use your definition, and call resigning and moving to another job "bargaining".