I don't recall that assertion, but I don't memorize every post.
I'm guessing this is me. I don't remember writing that it is impossible, just that you are more likely to find an ace in the first round and elsewhere it is rare.
From 2010-2015 fWAR pitching leaders. 12/20 1st round picks, 2/20 second round picks, 3/20 IFA, 3/20 drafted after the 2rd round. This year 5/20 have been drafted after the 2nd round, so I don't think it has changed. It can happen, but it is rare.