The .NET Core wrapper doesn't support this type of call where you ask for IDispatch at the same time you create the COM object:

C / C++:

IDispatch* disp;
CoCreateInstance(YourCLSID, NULL, CLSCTX_ALL, IID_IDispatch, &disp); // fails

Python:

disp = pythoncom.CoCreateInstance(YourCLSID, None, pythoncom.CLSCTX_ALL, pythoncom.IID_IDispatch) // fails

You must first get an IUnknown reference and QI for IDispatch on it, like this in python (which is what Excel/VBA does):

import pythoncom
import pywintypes
from win32com.client import Dispatch

unk = pythoncom.CoCreateInstance(pywintypes.IID('{64D6F9F6-6163-401A-82E6-C941CAF01399}'), None, pythoncom.CLSCTX_ALL, pythoncom.IID_IUnknown)
disp = Dispatch(unk.QueryInterface(pythoncom.IID_IDispatch))
print disp.SayHello

IMHO, python COM support should be modified to do this always...