Complete B.S.
Developers can install old versions of Xcode and still update the apps.
Sorry, you're the one who is full of it. The 3GS max iOS version is 4.2.1, and the last version of Xcode to support that min deploy target is Xcode 4.4.1. Apple now requires 64 bit binaries for all updates, which first appeared in Xcode 5.0. So there is no way to submit app updates to support the 3GS.