Interesting. I have the reference document, Version 10.5, October 2019, and it does not have that note. Though I see it in the online version. This service was just added in 10.5. It would be interesting to get clarification from SAG on this. Normally, “internal only” services are not documented in the reference guide. The note stating “you must not execute this service manually” is a bit ambiguous. Executing a service manually means something specific and different from using it in a FLOW service.
The main caveat about making manual changes to the generated connector service is that your customizations will be lost if you re-generate the connector service.
I tried to find info about managing auth via the RAD connector service generated, without editing the connector service directly, but could not readily find it. Someone else have a pointer?