Doom vs Transports and G2G

Right. My own gaming group has ruled that the Doom doesn't hurt units in transports but I was recently playing a game where we decided we would use the Adepticrap FAQ answer to see if it made much difference to our game, seeing as they have an inordinate influence on the seemingly lazy GW Faq'ers.

So the Doom doesn't fly Ryanair (didn't deviate massively from intended target) and does his thing on 2 units in tanks.

Adepticon says I get a 4+ cover being in a transport so I said I would go to ground to make it a 3+. We argued for a while before dicing off and I could go to ground inside my tank. He killed a few and the tank they were in (not gone to ground) drove off.

After the game we were wondering about this evil piece of rules lawyering on my part. There's nothing on pg 28 that says you can't do it from inside a tank but that's probably because there's nothing else that hits units in tanks if you're not playing with the Adepticon rules.

As GW haven't put out a FAQ but often follow the answers put out by this crowd I'm wondering how people will treat this. Hopefully GW will see sense and not go with it but if it does lazily follow rather than lead, how would you play the above example?