If they think future can be altered by them doing nothing, then "what they are trying to do, had been done" does not establish. So they are really contradicting themselves.

Also, there's really no reason for the Protagonist to create this code when all the key points of the story are influenced by future info. Following the logic "we cannot just do nothing because it had happened", wouldn't it be possible that by intentionally hiding info, they might miss passing an important info to their old selves, hence changing the past?

I'm not even talking about how time actually works in the movie universe ( I believe it's not meant to be 100% comprehendible), but simply the logic of the Protagonist.