Well spotted!
When I did a ZDU some days ago, I remember thinking "well, this Zero Downtime thing certainly must be for very large values of zero", so this name change is ... apt. :)
One of course can't help but note that only the header was changed. The text itself still refers in a multitude of places to Zero Downtime, ZDU, and "you can upgrade your AE system without any downtime".
p.s. both the inability to roll back the DB update, and the temporary nature of any roll back to a previous version was mentioned to me at AutomicWorld or similar occurences. So I can confirm that I heard it both said at some time right from the source.
p.p.s. I can also confirm, but this only from trying it and thus annecdotal evidence, that you can indeed not start old server processes anymore once the [MZ]DU update has progressed sufficiently.
p.p.p.s. The link you gave is part of the ARA guide, yet describes the AE. The actual AE guide still speaks of ZDU at present time,
as found here.