No, you can be an ML practitioner with just an intuitive understanding of, say, gradient descent works and you would do fine. You can even pick up that intuitive understanding on a strictly need-to-know basis, when it's needed for learning an ML technique. That's what fast.ai teaches.
For being more than a practitioner, like an implementer of new ML libraries or a researcher, of course you'd need to know more.
For being more than a practitioner, like an implementer of new ML libraries or a researcher, of course you'd need to know more.