You don’t get it. The point of the author is that you want to be in the position where a company is hiring you, as a consulting business, to solve a particular problem they’re having. That means having very large freedom in your implementation, as long as the problem is solved.

What you’re describing is basically being an employee. When you’re an employee you typically aren’t hired to solve a problem, you’re there to fill a role. To fill the role you will need to fit whatever mold the employer wants, including tech stack.