As for what specific compression format Portal uses, I dunno. I recall seeing a forum post once that implied it was using regular zip compression, because the file size worked out about the same, whereas 7-zip's unique format saved more space.
The first two bytes of the .ZAP16 file from this thread are PK, indicating it is a regular ZIP-compression archive file; Microsoft office .*x files (.xlsx, .docx, etc.) are the same.
I am only making the distinction between uncompressed/compressed archive
formats (ZIP, 7z, GZIP, XZ, TAR, TGZ, AR, RAR,
etc.) and archiving
applications (PKZIP, 7-zip, WinZIP, etc.).
I was sort of off the mark saying that 7-zip is not a format, .7z format is a format specific to the 7-zip application, but even PKZIP can at least read .7z format. According to the 7-zip page it saves about 30-70% better than ZIP, although that is probably with respect to the default ZIP compression level, 5 ("the best balance of compression and speed") on a scale of 0-9, so ZIP format can do better than the default.
Renaming a .ZAP16 file to .ZIP allows non-ancient Windows systems to open or extract the contents natively, whether or not 7-zip, PKZIP, or any other archive applications are installed.