Yeah it's possible though Eddy Cue did hint at a new ios 9 seed. Maybe B3 will seed today and 8.4 tomorrow to reduce server strain
They have announced that they have changed the way delivering software. Previously, Apple need to first download the package, unpack it, and write contents to iOS partition. Lets say an update requires 2.5GB to download, then, you may need at least 5.1 GB of free storage to store both downloaded file and extracted files. Now, I guess, they update software by
streaming software package or even files (just like how Office 365 software installs on your PC) to your iOS device. This doesn't require huge space for storing a whole IPSW package.
Therefore, simply to enable Apple Music, Apple only needs a very small file to enable or install related tabs and open access to Apple Music server, and everything would be fine. They don't really need to seed an update large enough such as 400MB to do this. And for iOS 8.4, the only major change is music app, which will not take much disk space to install, and will not even need to download a large file.