I think we are conflating a things here. NHS Digital internet facing tends to mean NHS Digital API’s will be internet facing. E.g. the SDS Endpoint API will be GET {uri}/FHIR/R4/Endpoint?organization?{odsCode}
That uri is internet facing.
At present those Endpoints will be for API’s normally part of a NHS Digital API e.g. GP Connect.
If this returns a uri, I’m not sure what the rules for this are. (internet facing may be a safe bet).
However you are talking application not API. So I don’t believe this applies. Your applications backend is likely to need to be on HSCN, SDS LDAP is HSCN only, newer interfaces like PDS I believe are internet facing (as would the SDS FHIR API).
The suppliers I’ve worked with have all gone for internet facing applications.