Hats off, Jonathan, for taking a lead on this important question!
It's my sincere hope that the Ubuntu community continues to be one of the most open and accepting open source organizations in the world.
The current one-line description of Ubuntu membership is:
Membership of the Ubuntu community means recognition of a significant and sustained contribution to Ubuntu and the Ubuntu community.
I, personally, wholeheartedly welcome any and all who have contributed to Ubuntu in any way, even if indirectly, to the Ubuntu community. (The words "significant" and "sustained", I think are too subjective for my tastes.)
Without question, to me that includes any Debian Developers, Maintainers, and countless Open Source Upstream Developers (even where those upstreams are employed by Ubuntu's competitors).
If such a person is willing to sign and abide by the Ubuntu Code of Conduct, they're an Ubuntu Member, in my book.
The more people in this (free software) world who abide by the Ubuntu Code of Conduct, the better place it will be for all!
An oldie, but goodie...