The rough distance from the Bellagio to the Mandalay Bay is 2km - I accept that you could very possibly hear the shots in the distance at 2km in some situations - however, there are many large buildings in between the two hotels which both block direct line of sight and will also act to reflect and heavily dissipate the sound vibrations from the gun at the Mandalay Bay.
When this is taken into consideration I find it unlikely that they would have heard the shots so clearly from inside a noisy/busy casino if they were indeed from the Mandalay Bay and not from somewhere much closer. You can see in the videos from the ground floor of the Mandalay bay that the people there were mostly totally oblivious to the shots coming from their own building, directly above them - so I find it unlikely that people 2km away would have been so freaked out as to form a stampede - unless there were, as the witnesses here state, 'bullets flying through the lobby'.
I agree that there are questions to be asked here and I find it a bit strange that there aren't more videos from the hotel showing broken glass and maybe bullet holes if that is what really happened there. It's a subject that warrants more investigation.
It is possible that the people in this video were misinformed and confused - we shall see.