I think it's a rather simple observation which teams players improve the most from the beginning of the season to the end. Especially with junior players that have such a high curve.
I'm not sure many would evaluate London at average to poor at player development.
It's hard to make an objective assessment given their huge number of late round hits on college commits that were already destined for the NHL, but here's a quick summary of how many top 3 round picks made any serious developmental strides with the Knights. The most borderline guys I left off were the likes of Billy Moskal and Doug Blaisdell, who were not obvious development successes.
2012: 0/4
2013: 1/3 (Marner)
2014: 1/5 (Jones)
2015: 2/3 (Bouchard, Thomas)
2016: 1/4 (Foudy)
2017: 1/3 (Perrott)
The list is littered with guys like Tyler Nother, Emanuel Vella, Mitchell Kreis, Ryan Bangs, Lucas Rowe, and a whole bunch of others who didn't even make the OHL.
Max Jones likely wasn't available to teams, but I left him anyways. That snapshot contains 22 top 3 round picks, meaning less than one third of top picks had any serious developmental success with the Knights.
There have absolutely been some non college commits in later rounds that have had success, most notably Brett Brochu, but it's hardly a group that sets them apart from other teams.
A couple of other teams for reference:
Greyhounds 10/15 ( McCann, Spinozzi, Speers, White, Senyshyn, Katchouk, Raaymakers, Gettinger, Hayton, Calisti)
Rangers 9/19 (MacInnis, Mascherin, Bunnaman, Hall, Merieles, Garrefa, Vallati, Damiani, Vukojevic)
Included in the misses were traded players like Anthony Salinitri, Liam Hawel, Hayden Fowler, Ben Gleason, Matt Schmalz, and Grayson Ladd.