On the one hand, it's hard to deny that a requirement (or strong preference) like this will often discriminate against some fairly large groups. And it feels cringe-y for that reason. But it's also hard to deny the value of a portfolio that's directly relevant to the job being hired for.

And, if I'm being honest... I do a lot of writing as part of my job. If I'm hiring someone for a role like mine, you can be sure that I want to see some writing that they've done. If they don't have anything they can share? (Which seems a little off. But maybe it was all internal company communication stuff.) Well, they're going to have to write something because I'm not going to just take them at their word that they write well.