Both of those recommendations have only one purpose: to increase sales.Two other pieces of perceived HiFi wisdom were
1. source first - spend disproportionality more on the front end. That is so backward in my experience , unless perhaps we are talking specifically about vinyl playback.
2. Spend 10% of your budget on cables
I routinely read and heard these silly things in dealers and in magazines. I am grateful to finally be better informed and as you rightly state, to have the technology to measure for myself in my room.
Still, if measurements would become the accepted way to decide on hifi electronics purchases then the whole business would crash and burn spectacularly. The rest of hifi shops would be out of business as well as most of manufacturers specializing in hifi electronics. The hobby itself would get pretty boring for most as you basically buy the best speakers you can afford and that's it. It might even be that people would quickly realize that after something like 1k€ the differences between speakers and subs are not that big in usual listening conditions.
In a way, to have this hobby we kind of need people believing in snake oil.