The internal market goes back much further than Cameron: 1989 under Thatcher with the 'Working for Patients' white paper that introduced the "Purchaser Provider Spilt".
navigator.health.org.uk/theme/working-patients-white-paper
Also in 1989, in preparation for the National Health Service and Community Care Act 1990, the Dept of Health published a document called "Financial Turbulence in the NHS". This identified nine low-incidence, specialist services that would go to the wall unless they were protected by special purchasing arrangements coordinated at Regional level.
They included Regional Genetics, Regional Drug and Alcohol Services, Specialist Wheelchair Services, Communication Aid Centres and "Gender Identity Services". (I can't recall the exact term they used for "Gender Identity Services" and I can't recall what the other four services were.)
These were often referred to as "Fire Station Services", ie. you are rarely if ever going to need a Fire Engine turn out but if you do it is important that there is one available.
The purchasing and then commissioning arrangements changed over time. Service Contracts (a fixed sum for an agreed level of activity, re-negotiated annually) were supplemented at some point (well before 2013) by Extra-Contractual Referrals (ECRs), ie. funding for individual patients, negotiated case by case, where there was no existing contract.
ECRs were "income generation", ie. income over and above Service Contracts, but they did not represent excess income over and above what was required to deliver the activity specified in service contracts.
That is, Service Contracts did not cover the full cost of delivering the contract and ECR targets were set as part of the contract in order meet full costs.
(I am putting all this in the past tense because while I worked in that system for 20 years, writing Service Contracts, doing annual Marketing Planning exercises and discussing potential ECRs with referrers, I have no idea if that is how the system still works.)
Anyway, this is the way that the NHS worked for decades and was certainly standard practice long before 2013, the date cited in the article.
All that the article "exposes", which I do not imagine was ever a secret, is that the Health Service in Northern Ireland in 2013 was using the same system of Extra Contractual Referrals as the NHS.
The only thing that would be surprising would be if other specialist health services in Northern Ireland did not rely on ECRs to top-up Service Contract funding in order to meet the full costs of those services.