Because this isn't one game. It's a consistent trend across 8 years where we have consistently failed to do something. The Dunning Kruger post was about hindsight and the discussion of tactics and how it could affect causality. This is about an overarching problem with how we approach bringing players through our youth system that has spanned near a decade. Many people misunderstood that as "well it's impossible to have an informed opinion on anything", of course it isn't. It isn't impossible to see that Sergio Aguero is our best striker because he has a record that can be referenced in comparison across many datapoints. It's impossible to say that Aguero would have been a better striker for a specific game than Bony because you're not working off of any data points alone in context of a single game.
Essentially, if you don't have any data to work from then it applies but if you do have data to work from that spans what is a relatively long period of time then you can form judgements based upon that. It's not necessarily correct, but you can at least have an actual opinion that is backed up.