As far as I’m aware Duo does not classify it’s self as an IDP and instead will integrate with whatever your current IdP is.
While looking into this a bit deeper there are some caveats.
e.g. When trying to protect Google Workspace (GSuite) you can’t also have google as your IdP.
So what IdP is everyone pointing to:
GSuite (Can’t protect GSuite accounts if this is also your IdP)
AzureAD (May requires P1 licenses and Conditional Access)
AD (May require DirSync and 3x recommended domain controllers)
And if you are going to point Duo to a separate IdP why not use their MFA tools (e.g. Okta/OneLogin)