There is also a matter of people knowing if the game song is a non-original piece in the first place. There has been a few times that I've heard classical pieces in video games, and im sure there has been many that I didn't recognize. I didn't know Snapplemans song was from anything other than a videgame (besides nutcracker obv) until it was pointed out by a friend who recognized the motif. In the case of someone who has never entered DoD, this could be pretty discouraging to have your song disqualified because it was part of Act II of some 15th century opera.
Summed: one cant tell if someone purposefully dodged the rule about adapted vgm.
And then, in terms of referencing another song vs. basing an entire song around it, half of my song was Hulk Hogan's theme from WWF. I counterbalanced it with a Casltevania tune, sure, but it did take up a decent chunk of my song. I know we all throw in musical references into our songs, I'm pretty sure my last 4 DoD entries have each had some other tune hidden in there.