This is true for the Bourne Again, Watanabe, and Korn shells too. The default mode is not the POSIX-conformant mode, but is the shell's native mode. This does

This is true for the Bourne Again, Watanabe, and Korn shells too. The default mode is not the POSIX-conformant mode, but is the shell's native mode. This does not, however, make these shells "not POSIX compatible" any more than it makes the Z shell "not POSIX compatible". In all of them, one has to do non-default stuff, from setting particular environment variables through invoking the shell with a particular name to adding extra command-line options, to pick the POSIX-conformant mode.