My guess is they ran into problems with testing machines that are running a 32-bit OS and/or are only running 1-2GB RAM. It still baffles me that people are using 32bit OSes these days.
The amount of memory in a system is physical memory, LAA/4GB tuning deals with virtual memory.
But yes, having less than 4 GB physical memory could cause serious slowdowns as virtual memory will have to be swapped between physical memory and hard disk.
For Skyrim, just enabling the LAA flag is not enough, because when you change the policy of windows memory manager to top-down, it will crash on the menu screen. This shows that there are at least some parts of Skyrim or it's 3rd party libs that cannot handle 32 bit addresses. Fortunately, this memory is allocated during start up, so with the default bottom-up memory allocation it is not a problem. But when you want to officially support it, this crash should not occur.
As a developper, I'd also want to know why the 4GB tuning is helping... since it is reducing crashes for people who are reporting that Skyrim never approaches 2GB of memory allocation for them. It could be memory fragmentation.