Nope one Client is the Server! Thats the big difference. And he can execute Server functionality aswell as reverse engineer sensitive Data. Or simply change Data thats sitting in Memory directly. You can simply say the server is on his Machine and he can Manipulate it.
On a Dedicated server no Player has any direct access since everyone just connects to the Server. Nobody owns it. It makes it also harder to hack if someone sends some shady package the server can verify it. Did not happen on the Server → refuse the package.
And all those nodes you mentioned are executed by the Client since he is the Server. He runs all the Functions and tells other Clients whats going on. See the Problem now? ^^