The browser downloads all CSS resources, regardless of the media attribute. Check your network tab on your site and you'll notice even on mobile devices it downloads styles-l.css.
The media attribute allows the user agent to pick the best adapted one for the device it runs on, in other words it will de-prioritise styles-l.css which helps prevent it from ...
This question has been asked before although in different ways.
What you are seeing is the same as everyone else.
Magento 2 doesn't score high in certain areas because of how it is structured. For example require js decides what content to show.