Before you say something like that, you have to specify what you mean by ”waterproof”.

Breathable fabrics are tested both against resistance to water pressure, and for the amount of water vapour they can release.
And technically, they do work. They can withstand enough water pressure to be considered waterproof in an everyday use of the word, and still be able release a certain amount of water vapour.
But usually, as water resistance goes up, breathability goes down.
So far, I haven’t found any garment that breathes well enough for mid/high intensity activities while also being waterproof in an everyday sense.
But ”waterproof” and breathable enough for low intensity activities, sure.

No need to do that on general principles.
”Avoid waterproof for high intensity activities”, I’d buy that though.
A better recommendation would be ”do your homework before buying”.
Look at the water column/breathability balance, find out what you’re actually buying.
The jacket that’s a good choice for fishing isn’t going to be a good choice for high intensity riding, skiing or running.