This is just regurgitated falsehoods that nobody ever actually looks into. It's the same echo chamber bullshit from /r/Noctor. Nothing prevents an MD/DO as acting as an expert witness and it is routinely done, this sounds like someone with zero medmal experience. There is absolutely a standard of practice for NPs, it changes based on state and practice environment because of the supervision structure, but often shares almost identical responsibilities with a duty to refer when outside scope.

https://expertwitness.substack.com/p/thoracic-back-pain-death-np?s=r

Literally the first medmal case I picked, EM MD as the expert witness for an NP acting in the ER.