I've had a reply from the platform owner:
"In order to measure the “time played” inside a game, we need a constant exchange between users interaction and tracking information send to GA. If there is no “Level Start” event inside the games, we cannot tell how long he is playing the game. This is due to the technical nature of how tracking systems are working:
Situation 1:
Open the Game (information send to GA)
Leave the website (no information send)
Situation 2:
Open the Game (information send to GA)
Start Level Event (information send to GA)
Leave the website (no information send)
Tracking systems will measure the time between two interaction points. When there are no follow up interactions after opening the game, the tracking system cannot measure the time in between and will return 0:00 time played for the user in Situation 1. Integrating events inside the game will give the tracking systems more interaction points to calculate the time in between those. The same user from Situation 1 (return: 0:00) will now return a time played in Situation 2."
Does it help? Not sure this gives enough information as they don't tell HOW it should be done. :-/