For PIP and ESA Purposes: There are a number of medical conditions - normally neurological in nature - that may only be assessed by doctors.
According to this https://welfareweekly.com/thousands-of-disability-benefit-assessments-deemed-unacceptable-by-the-governments-own-quality-audits/ IAS and Capita only had a grand total of 4 Doctors working on PIP assessments in 2017. Unless this has changed significantly I can’t see how they can be following this policy.
This has been a longstanding policy for WCA but, as far as I am aware, there have never been any rules limiting which HCPs consider PIP cases. As pointed out, it would be impossible to restrict any class of assessments to doctors only because there aren’t enough doctors doing the assessments.
Even if it were the case that the policy had been breached in your case, it would not have the effect of somehow rendering the conversion decision unlawful and it would only go to the probative value of the report.