As @molbdnilo mentioned in a comment, @@ refers to the path to the test input file, not its contents.

By default, AFL/AFL++ read from stdin. If you want them to use argv instead, you have to employ a small trick to get it to fill argv with stuff from stdin. You can find an example on AFL++'s repository: https://github.com/AFLplusplus/AFLplusplus/tree/stable/utils/argv_fuzzing.