Addicting sounds present tense to me, so I think that's actually acceptable what Muller said.
Addictive, to me, sounds future tense more than present tense, which would imply he's not currently an addict of the game, which is not the case.
I guess a past tense form of the word would be "addicted"... Though "addicting" would also work. "That game used to be addicting" or "I was addicted to that game", both are past tense I guess, but the first would be an adjective for the game, while the second is an adjective for the person...
Weird, I'm not sure how grammar laws dictate that.
This has been Professor Sterlin with your English lesson for 2 AM.