* I'm sure Microsoft's Windows developers all work on Windows.
* I assume Apple uses Macs for all development right now.
* If memory serves, Google uses a lot of Macs themselves, but did at least make an effort to make it possible to do development on Chromebooks.
* Oracle... it's not clear to me that Oracle actually has anybody working on Solaris, but if they do then they have to use Solaris to work on Solaris unless they've managed to completely overhaul the build system. For everything else, OEL should at least theoretically be completely adequate.
* I'd be interested to know what IBM uses internally; AIX was a workstation OS but I have no idea if they've maintained that capability, and they own RH now so that's an option if they wanted to stay "in-house"
* I don't have any specific knowledge about it, but Red Hat absolutely could use Fedora and/or RHEL for most everything.
* Canonical is almost certainly running Ubuntu everywhere.