It also seems to be wrong, since the ZVV website explicitly mentions the app, the online ticket store and vending machines. [1] What seems to have happened though is that during the pandemic, the bus and tram personell that used to sell tickets as well stopped doing so to minimize contact. [2] This obviously is still a problem since not all stops have vending machines, so there’s the extra rule that you can buy your ticket at the next stop that has a vending machine. Inconvenient? Certainly. End of the anonymous travel? Certainly not.
[1] https://www.zvv.ch/zvv/de/abos-und-tickets/tickets/einzelbil...
[2] https://www.zvv.ch/zvv/de/allgemeine-seiten/medienmitteilung...