I think, they are also not wrong, like experts have said such things for some reasons, and the statement of "Wait for the perfect setup everyday" is almost the same as what you are saying, that we should not restrict ourselves with specific hours and wait for the right opportunity.
Only thing that makes it look like restricted is the word "everyday" because no doubt a trader might not find the best opportunity even after a week but that's so rare and depends on the filter, like what actually the trader is looking for.
Although there is no doubt that, we must not restrict ourselves, but sometimes a limit is better and saves us from huge losses.
Yeah, when you start to trade everyday and you study how trading works, you realize that there is no "this is how you make money", because there is no single way. You can do one thing and make money today and a week later the same way would make you lose money, it's that simple and there is no way that you can get better at this.
The sad thing is that if you really want to make some money, the only way you can do this would be to learn to be dynamic and change based on how the market looks and focussing on session must be one of them. Still, adapting your strategies with respect to different session alone will not guarantee you profits, but could impact significantly like in bullion trading, staying away from market in the opening session of US market would help to avoid wild fluctuations.