Yes, and that's all nice. But what are the advantages? I, and probably most other non-technically minded folk here, really don't care what's not possible. Basically you're saying Toady has been fooled into wasting time making a 64 bit Dwarf Fortress which does nothing and for some reason wants us all to get excited about the nothingness. Great.
The *primary* advantage is that 32-bit hardware, and therefore software, is increasingly antique. x86-64 (by whatever name; AMD64, Intel64, etc.) was announced in 1999, and 64-bit CPUs for desktops have been readily available since at least 2004, and the default since around 2006. 64-bit adoption in other markets was even earlier; consider the Nintendo 64 of 1996 and the Playstation 2 of 2000. Windows Vista, around 2006, was 64-bit for serious users, and Windows 7, around 2009, pushed 64-bit operating systems and compilers further into the mainstream.
As DF goes forward, we don't want it to get so far behind that it slows down even further because it has to run on an emulator. There's traces of that already; read
"A bit of background on compilers exploiting signed overflow" for an interesting summary of the considerable overhead that using old-fashioned 32-bit loop index expressions can cause on a fundamentally 64-bit processor.
Moving DF to what's been the standard for computing for literally a decade, and taking advantage of modern compilers which have substantial optimization improvements, is a really good idea. My personal estimate is that we'll probably see a few percent to a few tens of percent improvement just from properly modernizing the tool chain; and that the cost of only supporting a terribly out of date setup would be climbing at an increasing rate over the next few years.
Addendum: According to the Steam Hardware Survey, over 88% of Windows gamers on Steam are using a 64-bit OS. Mac Steam gamers are over 97% 64-bit OS; Linux Steam gamers are much harder to tell because of the diversity of versions not being shown, but all five of the listed top versions, representing about 60% of users, are 64-bit.
This June, when (supposedly) Microsoft does one more big push to get folks on Vista and 7 to upgrade to 10 for free, the ~7% of 32-bit Windows 7 users will probably drop off further. (Around 97% of Windows 10 installs are 64-bit.)