German bus services are not publicly owned anymore and as private companies or conglomerates of companies they are free to implement whatever payment methods they see fit. I personally cannot speak to Nordrhein Westphalia or this particular bus company serving the airport but I had a similar experience in another state. The fare is significantly cheaper via smartphone (which refuses to install on Graphene) and the busdriver in question had no means to handle cash.

Ticket machines are still available, but only in highly populated areas or on locations like central (railway) stations. All others have been eliminated.

Yes, in many buses you cannot buy tickets at all (anymore), the ones who had like vending machines inside removed them, so you can often only get tickets via app, printing out at home, or at major bus stations.

The old ticket stamp machine still stood in the bus but it was taped over. The driver did seem to accept cash from some people and it seemed like random amounts (pay whatever you have?), and he did not give out tickets, hence my remark about bribing.

The app is called DB NRWay and has 1.6* on Google Play, so... See for yourself. I was surprised as well. Like I said: it was a surreal experience.

And yes: all bundesländer are _so_ different! NRW has another spiffy (creepy) app that uses geolocation from your phone to determine the amount of kilometers you have ridden on the bus, but it specifically only works within the bound of NRW.