Yes, it would be a problem for "retina devices that act like full size monitor displays." They would be violating CSS. But since -webkit-device-pixel-ratio
works for you, it sounds like this is caused by something else.
You probably omitted this:
The viewport meta tag is used in modern "mobile" browsers. (iOS/Safari, Android/Chrome, Mobile Firefox, Opera). It lets developers say "this is a properly-designed website, not desktop-specific crud". Without it, the mobile browsers assume your website is designed with an unspecified min-width, somewhere around 960 pixels.
When I say "pixel", I mean "CSS pixel". We've established that your CSS pixels are 3 physical "device pixels" on a side. And this means the largest dimension on your device works out at 640 CSS pixels. This is much less 960, so "desktop" webpages - which are assumed in the absence of a viewport meta tag - will start off zoomed out.