They're talking about an installer for Arch itself (which could and should support the bare minimum options that most users want), which is what it didn't have until recently; not installers for packages within Arch, which it has always had.

So, would it add a bunch of extra complexity? Not really; it's actually tiny compared to making a package manager and maintaining its library.

Would it take away from maintaining the package library? I guess a bit.

Would there still be a need for documentation? Of course.