From my experience in Uni P&T committee, the writing samples are part of the presented portfolio the candidate wants external reviewers to evaluate. The reviewers can't be expected to read anything else so it's up to the candidate to present a selection to make the best case for their promotion.

Unpublished work included in these samples is often downgraded by evaluators however. The undying faith of academics in peer review goes on unperturbed. (Not that I like it. Just stating what I have observed.)