You need consent for (not functionally necessary) cookies because of the ePrivacy Directive (the "cookie law"). Additionally, you also need consent for processing, storing or sharing personally identifying information (PII) because of the GDPR. Usually you do both in the same consent popup.
Plausible doesn't store visitor's IPs or any other PII, and doesn't set any cookies. The reasoning given in the quoted paragraph is incomplete, but the result is correct. You only need to mention them in your privacy policy, they don't require any opt-in popups
PII isn’t a concept in GDPR. GDPR talks about personal data, which on its own might not be identifying, but which in combination with other personal data can successfully identify a person.
Plausible doesn't store visitor's IPs or any other PII, and doesn't set any cookies. The reasoning given in the quoted paragraph is incomplete, but the result is correct. You only need to mention them in your privacy policy, they don't require any opt-in popups