That's a sleight of hand you're trying to do... the link explains how to load an app from an SD card, not how to load a 3rd party app (that is, an app someone else made and gave or sold to you independent of Microsoft). According to Microsoft[1]:
"To enable sideloading on a Windows 8.1 Enterprise and Windows 8 Enterprise computer that is not domain-joined or on any Windows 8.1 Pro or Windows 8 Pro computer or on a Windows RT device, you must use a sideloading product activation key. For more information about acquiring sideloading product activation keys, see Microsoft Volume Licensing"
So, true third party sideloading is limited to enterprises. Microsoft is just as bad as Apple in terms of controlling what the consumers do with their phones.
You are talking about sideloading in Windows 8/RT, not Windows Phone 8.
You can also sideload third party apps without a sideloading activation key in Win8, it's how testing on devices is done. You get a developer certificate for yourself (completely free and one button press when you first try to sideload) and essentially self-sign the apps. It's not as simple as double clicking, but it's one Powershell command (Add-AppXPackage IIRC). It's much easier to sideload with the sideload activation key, yes, because sideloading as a whole seems to have been created for Enterprise App deployments, but it's fully possible without it. I believe the only restriction is you need to renew the developer certificate every few months.
That's a sleight of hand you're trying to do... the link explains how to load an app from an SD card, not how to load a 3rd party app (that is, an app someone else made and gave or sold to you independent of Microsoft). According to Microsoft[1]:
"To enable sideloading on a Windows 8.1 Enterprise and Windows 8 Enterprise computer that is not domain-joined or on any Windows 8.1 Pro or Windows 8 Pro computer or on a Windows RT device, you must use a sideloading product activation key. For more information about acquiring sideloading product activation keys, see Microsoft Volume Licensing"
So, true third party sideloading is limited to enterprises. Microsoft is just as bad as Apple in terms of controlling what the consumers do with their phones.
[1] http://blogs.technet.com/b/privatecloud/archive/2014/02/28/i...