Did you copy the output right?  Testing your parser:

Without any arguments, I get the exclusive group error - the group is required:

0930:~/mypy/argdev$ python3 issue18943.py 
usage: issue18943.py [-h]
                     (--device-get-capabilities | --ptz-absolute-move x y z | --ptz-get-status MEDIA_PROFILE)
issue18943.py: error: one of the arguments --device-get-capabilities --ptz-absolute-move --ptz-get-status is required

0931:~/mypy/argdev$ python3 --version
Python 3.5.2

With one flag but not its argument, I get the error that you display.  That has nothing to do with the grouping.

0932:~/mypy/argdev$ python3 issue18943.py --ptz-get-status
usage: issue18943.py [-h]
                     (--device-get-capabilities | --ptz-absolute-move x y z | --ptz-get-status MEDIA_PROFILE)
issue18943.py: error: argument --ptz-get-status: expected one argument