He's wrong for the very reason everyone on here constantly states when they mention the PS3. Ultimately a multiplatform game has to make sacrifices that an exclusive title doesn't, I very much doubt Gears would have been the success it is being multiplatform originally. That being said I have no problem if they release Gears 3 on PS3 as long as they don't mess around trying to get equality in both versions, just get the 360 one done with its engine that will be tuned to perfection and then spend the time to get the PS3 version released afterwards.
One last thing I'd point out is that lag is a huge issue in Gears due to crap netcode implementation, I very much doubt given 2 networks to take into consideration the mp would have ever taken off it was worse than it is now due to not having the time to tweak both versions properly. Gears with a good host is heaven, when you get a bad one you pray to lose quickly and get the lagsaw out.