I think what he/she means is that core maintainers (or AUR maintainers) might be able to change a package without another core maintainer (or AUR maintainer) reviewing it.
Afaik, upstream packages aren't changed by Arch maintainers. It's the whole point of using Arch. And if you're using AUR, you'd better be checking the PKGBUILD (because no, it isn't verified by anybody). At the very least, you need to be checking where the package is being downloaded from.
And yes, Arch takes in the upstream vanilla packages, though there are testing phases before updates reach stable repos. I'm not sure any of this is a bad thing. It also means you don't have to wait 6-12 months for the newest version.