Making it self-destruct is the easy part, just change "DISPLAY=:0 xscreensaver-command -lock" to "sudo rm -rf /" or whatever you like. It's understandable that the author didn't want to put a destructive command in his example configuration.
At that point, the better option would be (IMO) to simply blow away the first 2 MB or so of your disk (where the LUKS master key is stored), run a "sync", and execute an immediate "reboot -f" (along with, perhaps, the other options that skip spitting out the warning message, writing an entry to utmp/wtmp or whatever, and so on).
There wouldn't be a real need to actually zero/wipe the entire drive (which would take a bit, even at SSD speeds).