Regarding your "PS," dare to dream, and I mean that in all seriousness. Esri doesn't even host the code for the API on GitHub, it is just a glorified check box, "we're on GitHub, check." The GitHub page isn't even accurate, in terms of its terminology, i.e., the API is "ArcGIS API for Python" and not "ArcGIS Python API."
At a minimum, I think this is a documentation defect. The documentation simply states:
extent
The extent of the geometry.
Sufficiently vague, it doesn't say an extent object is being returned, but it also doesn't say a tuple of values is being returned.
Beyond the documentation being incomplete, I do think there is a larger code issue. If someone wants an ArcPy Extent object, I think they should be explicit about it and use:
polygon.as_arcpy.extent
I think the Geometry objects—Common Data Types | ArcGIS for Developers documentation addresses this better:
Envelope
An envelope is a rectangle defined by a range of values for each coordinate and attribute. It also has a spatialReference field. The fields for the z and m ranges are optional. An empty envelope has no location in space and is defined by the presence of an xmin field, a null value, or a "NaN" string.
Overall, I agree with you, whether the ArcGIS API for Python team does remains to be seen.