There's not really that much of a difference between "not want" and "not afford". There are two levels of "can't afford". One is "completely out of my league", the other is "i could, if i made compromises elsewhere".
Owning a beaten-up low end smartphone for the utility (e.g. being able to check public transport times anywhere) is well within the "compromise" range for just about everybody, playing the "latest iPhone every fall" game would be clearly out of range for many.
The "going low-tech" crowd just happens to be very sensitive (and hostile) to signs of consumerism and this perception blinds out all traces of utility they might see otherwise. "I'm too smart to play a game i can't win" is a very desirable status message to send and it's best conveyed by an old Nokia. Being in Berlin and having a good time also makes them pretty immune to the nagging question of "am i missing out on something big by not having a smartphone?" that is driving smartphone adoption in other parts of society (e.g. the elderly).
PS: oh, and one argument in favor of "can't afford" crossed my mind nonetheless: some might actually think that they can't afford a smartphone. The really cheap options are not heavily advertised, so non-technical people in Germany often think that all smartphone ownership would be as expensive as the ridiculous "1€ iPhone" contracts they see on the billboards. Avoiding smartphone ownership because you are scared of hidden costs would be a very, very "german" thing to do.