It's a case of inverted causality. The author thinks that because a lot of great people he knows about wrote journals that writing journals helps make a great

It's a case of inverted causality. The author thinks that because a lot of great people he knows about wrote journals that writing journals helps make a great man. It may in fact be that great men that write journals are the ones he knows about. It does not address the (likely many many) people who keep journals but will not be considered great. It is a valid point that keeping a journal does increase your legacy as it becomes easier to write memoirs for your grandchildren. Even if no one else will read it, they will have a vivid portrait.