FAT12 is only 16MB, not 32MB.
Probably the author is checking the number of clusters which are:
For FAT12: 4086 (cluster size 0.5 - 4 KB)
For FAT16: 65526 (cluster size 2 - 32 KB) (not clear why he's checking with 512 then).
For FAT32: ~268,435,456 (cluster size 4 - 32 KB)
Details here: http://www.pcguide.com/ref/hdd/file/partSizes-c.html
Report the issue in that library ;)