This requirement mandates that the CPM vendor provide an Excel plug-in built on Microsoft's Web add-in technology, making it universal across different Office 365 applications.
Scenario: A non-profit uses the web version of Office 365 to gather budgets and provide reporting to their program managers in the field. They have no ability to manage software on individual PCs due to the distributed nature of the organization, as such they only use web-based tools.
Solution: To simplify the budgeting process for their distributed users, the non-profit deploys a new CPM tool on top of Excel for the web. This allows users to continue using a familiar, accessible interface to submit and review budgets. It also allows IT to centrally manage the plug-in for their users.
The most recent development framework for Office plug-ins is compatible with all versions of Office. If you are working with a newish (since 2013) CPM vendor who offers an Excel plug-in, it is likely that they developed it using Office Web Add-Ins, which is broadly compatible with all of Office 365 including Mac and Windows.