That's true, since of course developers need to be able to test their own applications, but that's hardly comparable to letting ordinary users install third-party apps, as you can on desktop Linux/Mac/Windows and Android.
Except for Apple's higher developer fees, you might as well say that iOS allows sideloading, then.
If an ordinary WP user wants to install an app that hasn't been published, they're out of luck, whether or not they have an SD card.
The criticism was "I can't install apps from third party sources". There was never a requirement to make it particularly user friendly in the OP. If it's a Windows Store application, then XAP sideloading coincidentally is easy. If not, then it's still possible, even if it requires you to be more of a power user.
Except for Apple's higher developer fees, you might as well say that iOS allows sideloading, then.
If an ordinary WP user wants to install an app that hasn't been published, they're out of luck, whether or not they have an SD card.