My twitter version on W8.1 Lumia 920 - 3.2.3.0 - no video playback. And when I go online, it gives me some other web page. If I go on the web with my W10 phones (1520, icon) they look just like the app on W10.
Maybe Twitter is not releasing a PWA interface for 8.1? I'm guessing.
I thought the point of PWA's was that anything the uses the web can access it.