not sure about this one really, but could it have something to do with the fact that the briefing.html is pretty much outdated in ArmA 2? According to the BIKI, briefing.html is only used for the debriefing, while for briefing you use the briefing.sqf.
Just a wild guess.
If you have ArmA installed and your briefing.html works fine there, try the very same briefing.html in ArmA 2. If it doesn't work then indeed the command seems to be kind of obsolete (along with the briefing.html).
But, like I said, this is mainly guess-work.