The CSS does include all devices. The quoted example is for a theme setting, fluid width, which is an admin control setting for the width of the layout by a percentage. It is set in the ACP and can only be fetched by PHP. It is not for responsiveness of a specific device. CSS natively handles that with media queries and included in all evaluated CSS.
At your level of high customization, if you’re only developing a theme for yourself, I don’t think there is a lot you would need to worry about that is being evaluated here as I imagine you don’t need a high degree of variability because you know how you’re going to handle your theme. So while I do get your point here that you lose this and many other expressions in the evaluated CSS, I don’t think that necessarily is detrimental to your theme. If it is, right now, you can use Designer’s Mode to review if they are detrimental to your theme or not.