GitLab team member here. With GitLab Ultimate, users with the guest role do not consume a seat. To learn more, see https://docs.gitlab.com/ee/subscriptions/self_managed/index....
I get why in some sense, because once you use ultimate you switch to using a EE install instead of the CE. But I think even having the choice between ultimate and normal paid tiers would be great, even if it means no free users on the instance.