It was, and still is, premature, given that the linked page says "More details on the scheme, registering your vehicle, and making payments will be published in due course". We don't yet know these details.
In particular, the FAQ that you quote fails to define any time limit for retrospective payment of the £5 charge, saying simply that a PCN will be issued "if no payment is received". So arguably, based on the preceding, even when you receive a PCN, you still have the option to pay the original £5 charge.
Contrast that with, for example, the London Congestion Charge whose
FAQs explicitly specify the deadline for paying the charge, after which a PCN will be triggered.