Confusing presentation on that release article. We don’t have formal GMs for point releases (as some have pointed out), and while the latest beta can become the de facto “GM,” today’s release is still a beta. The whole thing reads like a public release announcement.
I suspect that what happened was that 13.4 was actually due today, but because all the Apple stores are closed due to the pandemic and it is a LOT harder for people to get support, they wanted to delay the release another week to get it well-tested by devs and maybe public beta testers.
If an update breaks the OS right now it will be ten times as frustrating for a user to get service. Apple wants to be really, really sure.
I installed it on my phone, but I know how to roll back to an older release and restore from a backup. Your average person does not.