It certainly might be nothing, but it's hard to imagine they would say "all of their developers" are using Xcode to describe the current and diverse IDE landscape. A lot of developers are currently using multi-platform game engines, open technologies like HTML, multi-platform frameworks etc.
It really isn't, though. There's a lot of IDEs and text editors out there, sure (I seem to collect them, myself). But if you're specifically writing Mac apps in Objective-C or Swift, the entire development chain is tied to Xcode, and all the developers writing apps for Mac or iOS that I know of -- not "developers writing apps on the Mac," but the specific subset of people writing apps for the Mac or iOS devices -- are using Xcode. The only other IDE I know of that's remotely competitive on this front is JetBrain's AppCode, but it has a tiny market share in comparison, and people developing native Mac apps in other languages are an even smaller group.
(Also, it's not really like Apple to acknowledge competition in this space, especially anyone using non-native toolkits. They're not going to mention people writing Electron apps in the WWDC keynote, right?)
