You seem to be too generous, Lauda. Plagiarism, in my opinion, should be dealt with a perma-ban hammer at the first offense.
There is just no excuse.
For lower ranking randoms (most often trying to get quickly into sig. campaigns), sure. For established traders who didn't know better? No. I'm pretty sure we had cases like this one before (or at least one-time established member violations that didn't get banned; unless my memory is fooling me).There is just no excuse.
In this case however, it is not an isolated instance.
Correct.Yes, it does look unfair. The user is trying to edit his old posts and insert sources. 
So: Doing wrong = bad. Correcting wrong = Bad. 
He, obviously, started doing that after acknowledging the problem (after my PM) in this post. Would you rather that he left them as is? [1] Seems surprising that none of the DT users / mods have acted upon this so far.
There are internal moderation policies (although I'm not sure whether I'm allowed to share that; or at least what I remember of it). You should really take a good look at the user's posting history (outside of collectibles). e.g. Warning about BitConnect. [1] They are often not tagged, but just banned. Maybe neutral feedback (as long as it is not repeated again).