Apologies for the confusion and trouble, and appreciate the effort you took to troubleshoot, but as mentioned in the other post AGWA 3.x versions are created for ArcMap 10.x and not for ArcGIS Pro. Essentially, the answer to the question of "is it because the file format is .esriAddin and not .esriAddinX" is yes, that is the reason because .esriAddin files are for ArcMap 10.x and .esriAddinX files are for ArcGIS Pro. Also, renaming the extension will not work. The SDK for Pro is a different SDK than for ArcObjects in ArcMap, and the ArcObjects SDK wasn't brought forward into Pro.
It wasn't mentioned in the other reply, but the AGWA for ArcGIS Pro version will be released as a Python Toolbox.