This is such an annoying issue! They should just allow you to add both and choose one as the primary so the other redirects to it like every other website hosting platform my agency uses. Even Shopify does this - add the naked domain and the www subdomain and you can tell it the www is the primary so the naked domain gets redirected to it by Shopify.

They definitely need to update the documentation at the very least, that took us a while to figure out how we needed to do it. We had expected it to work like Shopify and all the others. But the real solution is adding this feature that should be there to begin with...