At the time that it was in the market, a "64-bit address space" wasn't the default. There's way more to it than the size of off_t, as it permeates the entire on disk filesystem structure.
For instance, the BSDs didn't have 64-bit block offsets until UFS2 which wasn't a default until FreeBSD 5 in 2003. In other systems, HFS didn't have 64-bit file lengths until HFS+ in 1998. NTFS started as full 64-bit, but you couldn't really get that on a consumer system until WinXP (maybe Win2000 for the prosumer market).
Nope, the change to off_t is literally all that's behind this claim. It's not Be's claim, explicitly, although they were of course in no hurry to clear it up.
BFS is specifically _not_ itself suitable for huge files. Last time I looked the default Haiku BFS settings won't allow files to grow beyond about 140GB. Which is fine for an ordinary file of course, but you certainly won't be happy if you expected to store a filesystem image _inside_ a file.
This happens because it's a rush job. For example there's lots of special corner case code needed for the extent handling in the multiple indirection case. I can easily imagine a week or two of work just to develop, let alone test, this part of the system. So, instead BFS although it labels these "extents" still just uses a fixed size block so that the code is simpler. It's not a problem at all, for small to medium size files on a defragmented or largely empty disk. But as files get bigger and disks get fragmented... oops.
NTFS dates back to 1993, it takes some real dodging and diving to conclude that NT 3.1 (in 1993) isn't a "consumer system" and yet somehow BeOS, available only to people who swore they were software developers who wanted to write software for Be's new BeBox architecture was.
For instance, the BSDs didn't have 64-bit block offsets until UFS2 which wasn't a default until FreeBSD 5 in 2003. In other systems, HFS didn't have 64-bit file lengths until HFS+ in 1998. NTFS started as full 64-bit, but you couldn't really get that on a consumer system until WinXP (maybe Win2000 for the prosumer market).