Ha ha
After quick consultation with my Corpo Travel Agency:
You are right - it is calculated as an an Open Jaw - 1/2 of GVA-MOW RT and 1/2 of MOW-HEL RT.
IATA fare calculation rules will never stop surprising me.
OK, I have found out that I didn't read the rule carefully enough so I understand now - the open segment must be shorter than the
longest flown fare component (instead of all flown segments as in the commonly used definition) - GVA-HEL is shorter than GVA-SVO so it is still a valid round-trip.