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.

I'm aware, I submitted an app already :-).

I'm confused about what he meant about the marketplace having a "bootstrap builder" for x-tags.

We're working on a bootstrap that you can download to quickly start developing app, and it will include x-tags (with all components). There will also be a "builder" where you can customize your version of x-tags to only include the components you want.

So the bootstrap gets you up and running quickly, and the builder is a web-based tool that lets you optimize your build of x-tags when it goes into production.