Based on these two comments though, there are plenty of non PSVAR scheduled coaches running. Thandi, New Bharat, Snap (partly debatable as it depends on the vehicle sent), Zeelo.Berrys has been mentioned a few times. There has to be a workaround with all routes being unregistered and operating under EU rules for long distance services (With zero passengers making trips of less than 15 miles). IF there is no workaround, these other companies would have been pulled in for it by now. There will also be paid corporate services out there which are non compliant.
There must be something to do with the 'separate fares' thing, which is what defines a scheduled service, and how it is being interpreted. Are they classing it as a private hire of a vehicle so trying to claim that the operator isn't taking fares, instead a 3rd party is (in this case, Flixbus is taking the money for fares and then hiring a bus off YJIP to convey the passengers). While it is scheduled, if not enough people use the bus it won't run and times are flexible depending on demand.
Of course I don't know how they are getting around it but I thought it only applied to registered services, none of the above companies services are registered so it was the only logical link that I could make.
I thought RRBs were done under separate legislation.