Arch itself is just a collection of user-produced PKGBUILDs. It's provided without warranty (like any GPL code).
The difference is that Arch got a trusted reputation throughout the years, while in AUR even the packages that also have a reputation can suddenly change their owner - which produced the current issue.
Nonbody removed any contributors. They just took away maintainer status from some people (rights to commit direcly to the kernel repo).
They are still allowed to contribute via the normal process.
They only removed him from the maintainer list, he can still continue to contribute normally (and he even says he will probably continue to contribute in the future commercially).
The difference is that Arch got a trusted reputation throughout the years, while in AUR even the packages that also have a reputation can suddenly change their owner - which produced the current issue.