This is specifically about extending the Apple Music streaming license to cover DJ software. If you already own your own music (DRM-free digital or physical), you can already use those tracks with your DJ software.
I've been able to mix together tracks from Apple Music (the service) I had on a playlist with wav files I've added in the past to... Apple Music (the app) straight from rekordbox and they worked fine.
I have been able to do this with Djay Pro since before this announcement. Not sure if it's now also on other platforms but it works great with Djay even for my own titles cloud synced through Apple music.