However it turns out that it might have been a bug. According to a variety of user reports, it seems that Apple has quietly fixed the issue because when the same question is posed to Siri now, it will tell you “I can’t do that for you. My apologies,” or “Surprisingly, that is not within my capabilities.”
This more or less confirms that the “feature” was indeed a bug with Siri. Unfortunately Apple has not responded to comments about the issue yesterday, nor have they come forward to acknowledge that they have fixed it either, leaving us to speculate as to why Siri might be prompted to call 911 when that command is issued.
In any case with the issue seemingly fixed once and for all, emergency services can now relax a bit knowing that as far as accidental 911 calls are concerned, there should be less as a result of this.