I stand corrected...

The documentation reads:
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FDO_API FdoInt32 GetGeometryTypes ()
Gets the FdoGeometricTypes that can be stored in this geometric property. The returned value may be any combination of the values from the FdoGeometricType enumeration combined via a bit-wise or operation.
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and it indeed mentions FdoGeometricTypes, and those are indeed encoded as powers of 2.
I was so focused on the name for the method...

Thanks for clearing it up!
Jörg