I can really only guess from previous info that they would not advocate thorough clinical assessments, in that they would not cover more in-depth testing such as brain scans, MRIs, immune system tests etc. It's my understanding they advised no in-depth testing for patients suspected of "cfs".
And, as you say OI, which in my own limited knowledge could be connected somehow to reduced brain diffusion, is in the BPS case, just viewed as deconditioning.
They are still advocating CBT and GET as treatments for ME. If they did investigate more thoroughly, they might come to the realization that "a chat and a walk in the park", won't fix ME.
I too thought they had changed their advice in advocating "thorough clinical assessments", but with further consideration, don't think anything has changed.
ETA: this seems such a circular situation. If GET/CBT advised for ME, then no in-depth biomedical investigations. If in-depth biomedical investigations, then no GET/CBT for ME (hopefully!).