But I don't believe BRW encountered any assassin ( however I'm not 100% sure about that.) I believe BRW encountered a "deputy sheriff" or a "security guard" (recall that Brennan said that the man with the rifle as wearing khaki colored clothing) That "deputy Sheriff" with a rifle, would be very intimidating to a young black man like BRW. If that "Sheriff" told BRW to get his ass off the sixth floor and forget that he had seen the "sheriff's deputy" there.
I agree with the basic scenario here: the sixth floor was kept off bounds by a person or persons posing as law enforcement of some sort.
However, I believe Mr Williams was NOT the only innocent worker to be kept off the floor. This would not have been left to luck. The 'decision' of none of the other employees to choose 6 is both odd and telling.
As for what the credentials-flashing man/men said? Instead of something threatening that would be apt to lead Mr Williams (& co.!) to believe something nasty was afoot (thereby raising the risk of an alert being raised), it would more likely have been a simple official instruction.
It was only when the assassination happened that Mr Williams------------and any other employees who had been kept off the floor-------------will have realized with horror that that man or those guys up on 6 were NOT there to protect Pres. Kennedy.
And if Mr Williams and/or Co. talked about all this afterwards? Not a problem. There was no attempt
by those behind the assassination to have this pinned on a lone gunman acting alone.