Everything from the 'main' repository has 5 years of support, you cannot load xubuntu-desktop (or any other desktop) from 'main' as it comes from 'universe' (community supported), so those packages are not supported with the five years of support.
Downloading Xubuntu does include packages with 5 years of support, but it's ALL covered by 3 years of LTS support (only some packages have the longer support life). We know parts have longer support, but it's a potential risk to use a system only partially supported, so it's safer to treat is as a 3 year fully-supported product (as advertized).
Refer to https://help.ubuntu.com/community/Repositories/Ubuntu for information on repositories & software-sources.
The moment you add PPA's you've made your system more complex (you do the security checks for PPAs yourself, and have to check your support completely yourself as some have no support).
The initial 'full-support' guarantee applies only to what's on the install-media (which may differ the moment you starting adding new sources). Ubuntu has 5 years of support for everything on install media, where as flavors have the shorter (usually 3 years) support.