The upper end of the Pirates' system has been quite productive this season, and with low draft picks in all of the last three years due to their big-league performances, the Bucs probably shouldn't be dominating lists of the top prospects in the low minors. Also, their last two top picks, Kevin Newman and Will Craig, were college hitters who were too advanced to be recognized on these lists.
When Wilbur points to the relative dearth of talent in the low end of the Pirates' system, though, things like these lists are what he's talking about, and it's worth watching going forward. In recent seasons, the Bucs arguably haven't compensated for their very low draft picks by drafting particularly well, and they certainly haven't taken advantage of the opportunities available to them in Latin America, creating a bit of a talent void.
By the way, one feels for the fans in Bristol, which is the Pirates' Appalachian League affiliate. When I visited in 2014, at least some of them seemed very invested in whether the team was winning or losing. Three years in, the team there has yet to post anything resembling a winning record, and the closest it's been to having a clear prospect for any significant period was Austin Meadows getting 17 plate appearances in 2014 or Mitch Keller and Gage Hinsz each making a handful of starts last year. When the Pirates made a deal with Bristol, they were adding an additional affiliate, not moving an old one, and that looks to have mostly been a waste of time.