Read the rest of the email you linked, where Linus clarifies that systemd's use of the 'debug' parameter seemed to assume that systemd was the only user of it, thus going against the whole point of a readable /proc/cmdline. Thus, feedback loops were caused and pain was created and flamewars were started.
Again, if you shove your hand into a fire, don't blame the hand (or, for that matter, the fire) for your own arrogance and stupidity. Common bloody sense.
If you discount the usual Linus style hyperbole from the rest of that mail, the only thing that remains is that Linus is unhappy with the fact that Kay closed that bug report instead of trying to find the root cause of the reporter's boot failure.
Again, if you shove your hand into a fire, don't blame the hand (or, for that matter, the fire) for your own arrogance and stupidity. Common bloody sense.