is always a valid command, whether file.name exists or not. When the file doesn't exist, tar will exit with status '2', apparently, but that has no bearing on the validity of the command.
Compare these two logs:
$ tar xvzf read.me
tar (child): read.me: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now
$ tar extract read.me
tar: invalid option -- 'e'
Try 'tar --help' or 'tar --usage' for more information.
Do you really not understand the difference between "you told me to do something, but I can't" and "you just spouted some meaningless gibberish"?
The GGP set the benchmark at "returns exit code 0" (for "--help"), and even with XKCD, the term in use is "valid command" which can be interpreted either way.
The rest of your slight is unneccessary, but that's your choice to be nasty.
Compare these two logs:
Do you really not understand the difference between "you told me to do something, but I can't" and "you just spouted some meaningless gibberish"?