Part of the success in getting this into Debian may be the result of there being no xz maintainer there. It is "maintained" by people whose attention is normally elsewhere doing occasional non-maintainer updates.
This code will have been running on the machines of a lot of distribution maintainers. If it has already been exploited, it could be that its real purpose has already been achieved and the real problem is now elsewhere. I sure hope somebody can figure out a way to determine if this backdoor has been used.
The multi-front nature of the attack, including multiple efforts to get the malicious code installed more widely more quickly, suggests we're not just dealing with a lone sociopath. I fear we'll never know who was really behind this, but I would sure like to.
There is surely more where this cam from.