I think the truth of that statement very much depends on the genre, workflow, and age of the composer.
Yes, some older composers cut their teeth with pianos, staff-paper and pencils and established workflows based strictly on getting the whole song on paper before the orchestration/arrangement, and still work that way today.
However, I'd argue the vast majority of younger composers and people who write in more electronic-influenced genres definitely do not follow that workflow. It'd be very hard to write an entire pop/rap/dance track that relies heavily on the drum and bass interplay by first writing it all out on piano.
Yes, some older composers cut their teeth with pianos, staff-paper and pencils and established workflows based strictly on getting the whole song on paper before the orchestration/arrangement, and still work that way today.
However, I'd argue the vast majority of younger composers and people who write in more electronic-influenced genres definitely do not follow that workflow. It'd be very hard to write an entire pop/rap/dance track that relies heavily on the drum and bass interplay by first writing it all out on piano.