Can this app work outside of 365?
Nope.
But it is a problem because many of us in corporate-land are barred from suing any of the plugins from AppSource--like the new M365 Esri for Excel.
Which is a huge bummer cuz he use it for a very common workflow that is a pain in AGOL otherwise. Plus, a lot of cool stuff in the M365 version!
ESRI actually responded to my questino about that...but I think there's no way they are going to support 2 sets of anything for these plugins.