The article tries to claim that we should take those claims with a grain of salt because people have been complaining about kids' literacy rates since time immemorial. The difference is that now it is measurably true.
I also think that anecdotal evidence is evidence nonetheless. If teachers active for many years start to ring the bells they are probably onto something, even if the official statistics are yet to come.
The author of the article starts by acknowledging a historical bias that generations have apparently always had against each other. They then explore, keeping an eye on their own potential biases, the evidence that literacy genuinely appears to be lower in younger generations. I don't know what more you could possibly ask from them.
I also think that anecdotal evidence is evidence nonetheless. If teachers active for many years start to ring the bells they are probably onto something, even if the official statistics are yet to come.