(1) Tightly coupled
The role of the CoS can be seen as being to support their principal/company, and so performance of the CoS is not judged separately as an IC, but is tightly coupled to and matches/mirrors the performance evaluation of the company/principal.
(2) Subjective
The role of the CoS can be seen as being to support their principal (and their leadership team), and so the performance of the CoS is evaluated by subjective feedback from their principal (and perhaps their leadership too)
(3) Delivery of things
The CoS has specific deliverables (eg special projects) and so performance is judged by whether or not (or how) those things/projects are delivered. This is the model Tyler uses in that article above.
(4) KPIs
The performance of the CoS is evaluated in relation to a specific set of measures/metrics
See for example the performance indicators of the CoS team at Gitlab:
https://about.gitlab.com/handbook/ceo/chief-of-staff-team/#performance-indicators
which include "the number of meetings shifted from CEO" - the number of hours the CEO spends in meetings should decrease overtime as the COS steps in)