"..Мы неправды тогда говорили,..."
In this case неправды is not the genitive of неправдa, but the plural of this word. I don't know whether it is a dictionary-word or Митяев invented it himself, but the poet means неправды=a lot of неправдa.

There is much irony and sarcasm in Митяев's songs, so don't take his lyrics litteraly.