You actually can screen print even wool knits, but you need special etching acid and then waterbased inks that penetrate fibers.
Not that E. used to work with one of the best garmentdye brand in the world, but sadly SI still use cheapest and crapiest plastizole screen print on it's t-shirts
But it's polartec syntetics, you can print it like in 3 different ways and it will be miles better. it's the same process sport brands use to print leggins.
this one is just lazy