Awaiting further information.
Without any doubt this is the correct answer. There is no information as to why they are there to start with so when he opens the door with an AR (in hand?) it is understandable and likely meets Terry for "officer safety" to take control of it. Since it did not appear that he walked out the door and closed it, no one can say that the officer did not see something that developed RAS. Without a doubt if as someone said they were called regarding investigation of disturbance they will use exigent circumstances to weasel out of it as well.
One thing however that is notable, when the officer is directly in front of the camera he is clearly unzipping several things. I am going to GUESS that it is a back pack or other container that the guy had in his hand or direct control. If not, I am not sure that would be covered. RAS allows immediate area but I would think probable cause would need to be met to start opening containers.
I do wish sometimes folks would remember the fifth amendment and just hush though. You state you want them to leave, you verbally deny access and then you request a lawyer prior to questioning, that does not mean they are going to bring one to you, it means they have to get you one before interrogating you.
I suppose everyone reacts differently but proclaiming over and over that something is "highly illegal" and a violation etc over and over is irritating to hear. Legal, illegal no high or low to it and there is no reason to try and debate a violation of your rights, there is no notification requirement for an officer to be in violation and almost every criminal they arrest screams it.
In the end, the entire event is questionable and there is not enough information to say otherwise.