The proof of brain idea about Steemit bis about adding value to it by creating something new and unique for the platform.
It is explained here:
https://steemit.com/steemit/@anyx/the-double-spending-problem-on-steemit

He is welcome to cross post his content and earn. Nothing wrong with that but he is likely to be detected by cheetah.
Community should be informed if someone created content solely for Steemit or just copying it from old writings.

He can just simply ignore the cheetah comments.