It is in the docs, it's just worded more clearly in a not so obvious place here which says
An external user is priced at the same rate as an end user on the Team and Business plans.
There is also Billing and usage | Retool Docs that states in the note
Organizations on the Business plan can restrict certain users from building or editing apps and workflows using permission groups. This would ensure those users are only billed at the reduced end user rate.
not too sure where you're finding silence, as there's End Users vs. External Users Pricing And Workflow, End Users Feature - Unusable! - #8 by Nicolas_Le_Jeune and How to create end user with Team plan - #5 by Kabirdas. all of which have a member of the retool team dutifully replying