Correct, Mozilla Marketplace is not putting X-Tags in all apps. The dev tooling side of Marketplace will offer a project builder that let's you pick which tags (if any) you'd like to include. It then bundles all the assets you pick into one compressed file and spits out a zip containing a skeletal project with all the script tag and asset references preset for you in the pages.

The Marketplace is just the "app store" -- it's where you buy apps. You can run the apps on any supported HTML5 device after buying them. So the supported device would not provide anything more than a browser rendering engine (and some navigator.* JS methods). It won't include x-tags.