In a coherent way, Esri delivers Location Analytics capabilities, data and geo-processing services through a platform.
This has a single resource center known as a portal. The portal becomes the CMS system for all Location (or Geography or Mapping) content and services for your organization.
Users (humans) or Apps (plugins) or Servers, all use and feed from that system (the portal). The portal can be implemented as an ArcGIS Online (cloud version) or Portal for ArcGIS (on-premise version).
So in no way, this portal replaces the role of your SharePoint as a CMS. No. And your SharePoint as a CMS cannot either fulfill the roles of the Esri Portal. There is no duplication or redundancy here at all.
Esri Portal is for your Mapping Business and your SharePoint is just a terminal to the portal from that mapping resources perspective while it still remains your CMS for your other businesses as you meant it to be.
In the same way your SAP, your office, your CRM etc. are just Apps (terminals) for the Portal; they render all mapping capabilities into those business systems of yours.
I hope this helps you to understand more the architecture.
Akakpo Agbago