The manual page is missing spaces before -- separators in the synopsis
section. The examples later in the manual page make it clear that is
incorrect and a space is needed in the shell before -- separators.
$ man parallel | grep -C1 options
SYNOPSIS
parallel [options] [command]-- [argument ...]
parallel [options]-- [command ...]
$ man parallel | grep -- --
parallel [options] [command]-- [argument ...]
parallel [options]-- [command ...]
If no command is specified before the --, the commands after it are instead run in parallel.
parallel sh -c "echo hi; sleep 2; echo bye" -- 1 2 3
parallel -j 3 ufraw -o processed -- *.NEF
parallel -j 3 -- ls df "echo hi"