I think what they're saying is in the times of Mozart, Beethoven etc you would not have been able to survive exclusively or mainly as a composer for piano. A career like Chopin's was only possible later on when music and the arts in general was more established and composers didnt live or die by royal patronage and the odd subscription concert here and there. Chopin is unusual in comparison to those earlier composers but not really unusual for his time. Liszt is another contemporary who wrote mainly for piano and there are probably at least few other lesser known 'specialists'(none of which come to mind alas)