64 Bit has only one advantage: A application can by default use more than 2 GB of RAM.
This is false. First, 32bit allows 4GB of ram per app not 2. 2GB is the default cap on windows XP, which can be raised to 3GB with a registry flag on winXP, on 64bit versions of windows it is 2GB per app unless a flag is set on the app to allow it 4GB (it is trivial to alter any existing EXE file to have this flag turned on. There are programs that will do that for you)
Furthermore x86_64 adds new components to the CPU that cannot be used by a 32bit program which result in a theoretical benefit from 0% to 500% increase in speed depending on the application in question. Actual tested benefits have shown ideal applications to get over 400% speed increase although that is very rare. 20-30% improvement is pretty normal though.
There are also security improvements but those are irrelevant for a game.
I think his point is that everyone has a 64bit CPU nowadays, and a majority has a 64bit OS. And co developing both 32bit AND 64bit means spending more money then developing just one or the other. So by hoping they drop 32bit support he is hoping for:
1. Proper 64bit implementation
2. Improved gameplay due to higher ram expectancy.
3. More money invested elsewhere in game rather then in making a second legacy version for 32bit.