Even if true, which I very much doubt, the fabric looks far too thick to be acoustically transparent. There will be reflections inside the cups (I'm guessing a warmish/dark bias). It's still a closed headphone...
I'll have to open mine and see it for myself to be sure, but looks like a plastic imitation to me...
https://www.head-fi.org/threads/philips-fidelio-x2.723025/page-675#post-12377055
But it's still closed; it sounds closed. In the small space of headphone earcups, a fabric that thick can't be transparent. Nothing wrong with that, manufacturers just shouldn't be trolling consumers. I'll probably rip it off if I get too lazy to return mine to amazon.