Is it true that, you can't be good/professional network security if you do not know how to hack the system?

No you don't.

You need to think like a hacker, know how they find ways to hack into systems.

What methods they use, or what tools they use.

Do don't need to know how to use the tool, or successfully hack a system.

You just need to know that you could be hacked using that service, or that method, then know how to protect it.

The end goal is to protect the server or service by thinking like a hacker, and not to hack it.