If you can't understand the proofs, then you're taking what this educator says on faith. You may also have a less sophisticated idea of when to apply which methods. Your ability to evaluate new results / methods / etc may be compromised by your inability to evaluate them in a principled way, which may be facilitated by your understanding of their underpinnings.
On the other hand, it's a rare work day when you derive a significantly new method / actually leverage the proofs / their underlying methodologies.
All in all, it's like saying "you can do software engineering without understanding theory of computation". You totally can, and can do it well, but you'll have some blind spots that won't be able to efficiently address / speak to your peers about.
There's no one right answer. There's the right answer for you.