Nah, zip can use several different compression routines. The only ones supported by zlib (and most zipping programs) are store, implode, and deflate. Deflate provides the best compression currently. However, there are two other less-commonly-supported encoding schemes: BZip2 and Deflate64. A few programs support Deflate64, such as 7-Zip and PowerArchiver, and this is most likely what WinZip 9 will have. zlib does not currently support it. 7-Zip can also use BZip2 encoding in zip files, but I don't think anything else can decompress those at this point in time.
Having Deflate64 support in WinZip will be a good thing, IMO, because it will become more common. Hopefully this will provide the impetus needed to get the zlib team to support it.