You may be right about certain Casino Protocols being in place as to who and when anyone is to call the cops.
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But get in a fight at a gambling table - with tens of thousands of dollars in chips around - and see how fast security gets there. I think you might agree "real fast."
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The Casinos I've been to have Armed Security....and lots of them. Casinos make a lot of money and the safety of "their money" and "their gambling customers" is Priority #1. The last thing a Casino wants is a "crime scene" that closes their betting tables or slot machines. 
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But "Casino Protocols" must include "what to do" when one of their own security guards radios that he has just been shot. If they don'y call 9-1-1 right away, then they better be sending other security guards (hopefully armed) to the 32nd Floor right away....as in right away.
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That 6 minute gap is "troubling." The 10 minutes The Shooter then spent shooting 600 people is "troubling." 
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That is 16 total minutes that I find "troubling."