What exactly do you mean by "treated like mercenaries", what should be the treatment in your opinion ?
In general, countries do hire mercenaries/private military contractors/etc, and it is not considered anything special, and many powerful countries (including e.g. the USA) routinely use mercenaries in their campaigns. the "sending" nation may restrict their people and companies from mercenary actions abroad if they choose to, but if e.g. Israel is okay with their company hiring out as a "mercenary" (the term usually implies directly participating in a conflict while being armed and excludes any other support such as training, logistics, software, etc, but for the sake of argument let's assume it applies here) for Saudi Arabia then there would be nothing unusual about that - for example, Saudi Arabia has used thousands of mercenaries in Yemen.
If the specific individuals commit something that's a crime in USA then USA can try to put them on trial, but that works exactly the same no matter if they're Saudi citizens working Saudi government or serving in Saudi military, or foreigners contracted out to Saudi government as "mercenaries"; in both cases it's up to the local government whether they want to hand them over (effectively betraying their own "employees") or refuse.
Presumably the GP means that the NSO employees should be held personally responsible for actions taken by the NSO group, without diplomatic cover or ability to claim that they're law enforcement/military/intelligence. They are civilians, not uniformed government employees.
My point is that this status does not influence those aspects, it does not matter if they are or claim to be law enforcement/military/intelligence, the practical consequences are the same.
They can be held personally responsible in USA criminal courts no matter if they're civilians or uniformed SA government employees - if a foreign government agent does something on your soil, you can (and should) apply standard criminal law can no matter if they're an uniformed employee in their service or not - for example, the Russian officers UK charged with Salisbury Novichok poisonings. However, USA courts can't enforce any judgements without cooperation of the host countries.
And Saudi Arabia can arbitrarily ignore the victims' complaints, foreign charges and convictions and their enforcement if they want, no matter if the violators civilians or uniformed government employees, that's only a difference if SA chooses to make that distinction. Uniforms would imply some differences in their rights according to Geneva convention if they would be captured as prisoners of war in an active armed conflict, but this is not an active armed conflict and they have not been captured as PoWs.
With respect to extradition or local prosecution Saudi Arabia can arbitrarily extend their protection to whomever they choose to, no matter what their status or citizenship is - if they have not made e.g. a bilateral treaty with USA where they agree that they will extradite such people, they do not have to do so.
In general, countries do hire mercenaries/private military contractors/etc, and it is not considered anything special, and many powerful countries (including e.g. the USA) routinely use mercenaries in their campaigns. the "sending" nation may restrict their people and companies from mercenary actions abroad if they choose to, but if e.g. Israel is okay with their company hiring out as a "mercenary" (the term usually implies directly participating in a conflict while being armed and excludes any other support such as training, logistics, software, etc, but for the sake of argument let's assume it applies here) for Saudi Arabia then there would be nothing unusual about that - for example, Saudi Arabia has used thousands of mercenaries in Yemen.
If the specific individuals commit something that's a crime in USA then USA can try to put them on trial, but that works exactly the same no matter if they're Saudi citizens working Saudi government or serving in Saudi military, or foreigners contracted out to Saudi government as "mercenaries"; in both cases it's up to the local government whether they want to hand them over (effectively betraying their own "employees") or refuse.