The CD mastering software they used to make the PC-Engine version of Art of Fighting didn't zero out the CD image prior to writing data, and as a result big chunks of unused space from the hard drive of the computer being used were included in the CD image. The data consists of files that had been deleted in the past including many archives of backed-up source code from various projects.
The data files and an extraction program somebody wrote have been around for a few years, but I think nobody found the archive that had the AOF code grouped together specifically until now. Data from AOF and other CD games can be found on this page on the top link, which includes extraction tools to pull out the fragmented LZH and ZIP files:
Or, you can obtain the ISO for the PC-Engine version of AOF and extract the data directly from there.
The developer of port had the AOF source code as it was a literal line by line port of the original game, the Hu6280 source has the 68000 code in comments and the corresponding 6280 code beneath it.