The structure is similar to this:
His promotion to captain should be coming up soon.
This has the meaning of:
His promotion to the rank of captain should be coming up soon.
This implied "the rank of" or "the position of" is generally omitted since it is understood. We wouldn't just put the article in, because it would change the meaning slightly. However, compare these:
I am not a captain, so please don't call me Capt. Rodes.
I am not the captain of this ship.
I am not captain of this ship.
All of these are correct. You'll notice that the article in the last two sentences is optional.
Now, the meaning in your quote is slightly sarcastic. The writer is calling Putin (presumably it's Putin) a dictator, when he calls himself a president. The suggestion is that he got himself elected, and then spent several years figuring out how to dismantle the democratic process and consolidate his position at the top. The writer is saying that he "graduated" from this learning process with the rank of dictator, and then gives a number of facts to back up his point of view.
I'll also suggest you read the post that user3169 linked in the note above, because it gives further explanation of why we don't use the article in this sort of situation.