I read somewhere that Lubuntu is developed by the Lubuntu Community, but Xubuntu is developed by the Community (i.e., community maintained). How are these two communities different?
There is the wider Ubuntu community, and within that community smaller communities.
The Lubuntu team can be considered of team members (https://launchpad.net/~lubuntu-members) and those helping the team (that may become members themselves in due course).
The Xubuntu team likewise consists of team members (https://launchpad.net/~xubuntu-team) and those that may themselves become members in due course.
Some Ubuntu community members help all teams, or help some (be it their favorites, or those in need of their skillset, or whatever they decide).
Becoming a member of a flavor is the same including need to show sustained and significant contributions so I won't include links as each flavor has their own slightly different page that is essentially identical. When you become a member of a flavor, Ubuntu membership is included (if not already attained, ie. Lubuntu team members are not separate from the Ubuntu community as a whole, just a group within the wider group which includes Kubuntu, Xubuntu, Ubuntu-MATE, Ubuntu-Studio, Ubuntu-Budgie, Ubuntu-Kylin [all flavors]).
The community support the 'universe' repository, which is where all flavor packages are found. Long term support (LTS) flavor releases have generally only three years of support compared to the 'main' repository which is for the most part maintained by Canonical employees (having five years of support for LTS release, longer via use of ESM) https://help.ubuntu.com/community/Repositories/Ubuntu
I've only used Lubuntu and Xubuntu as examples because you mentioned them in your question, the same applies to all flavors & smaller communities within the wider Ubuntu community.