I've read that online too.
But the person on Knit and Crochet Today (Lily Chin) who said she worked in the garment industry since she was 13 said it works better on acrylic than wool. Seems to be a lot of conflicting information/opinions about steam.
Personally I want my stuff to be limp. The stiffness of acrylic would be the reason I would make a sweater out of natural fiber rather than acrylic.
I tried it on a swatch (I learned ) of Lion Jiffy and yes it made it limp, but it also worked great. It flattened the stockinette stitch right out.
I may avoid putting the steam to the ribbing of that sweater, I'll have to make a swatch (I guess the learning stuck ) to see how it does on the ribbing.
I also tried it on the corner of a crocheted Red Heart SS afghan I'm doing (which is already limp enough that it really doesn't need anything) and I couldn't tell a difference.