Let's presume that the PM has issued a termination certificate under clause 90.1, which means that the Contractor then does no further work under clause 90.5. So even without the instruction under P2 the Contractor should do no further work, and if he does then this is at his own risk as he may not get paid for it as he'd be in breach of contract for doing so.
You should note that P2 deliberately refers to the Site and not Working Areas, therefore if for example the Contractor's site establishment and materials storage areas are outside the Site but within Working Areas the Employer's authority does not extend to these areas.
From an Employer's point of view, you may have another contractor lined up to complete the works, in which case you would want to ensure the Contractor had left the Site so as not to disrupt the incoming contractor.
P2 contains three elements which the Employer may instruct:
(1) leave the Site
(2) remove any Equipment, Plant and Materials and
(3) assign the benefit of any subcontract
This gives the Employer the flexibility (use of the word 'may' means he can choose to do it or choose not to do it) to respond to the termination in the way that best suits his plans.
By not instructing the Contractor to leave the Site, the Employer is allowing the Contractor to leave when it suits him to do so, which may not fit with the Employer's plans.