I only see two things addressed by "exigent circumstances".
1) Providing an excuse to enter a dwelling without a warrant. (i.e. Officer believes homeowner may be in danger and cannot grant permission. If I am having a heart attack, I do not want cops to wait for a judge to issue a warrant before they kick the door in and provide first aid.)
2) Providing an excuse to not announce their affiliation (i.e. "no-knock" warrants where they can be reasonably assured that if they announce their affiliation - "Open up! IT's the police!" the reason for the warrant will be destroyed - flush the pot down the toilet.)
The link you provided stated that the government cannot manufacture its own exigency (i.e. launch a flash-bang or CS cannister into the house, then enter because of fire, or danger to the homeowner)
I fail to see how, since Mr. Hart was already in custody, they could use "exigent circumstances" as an excuse to sieze his firearms. How would siezing them prevent bodily harm? He was in custody. I would think it would be impossible to figure out how you can enter a dwelling without a warrant after you have the suspect in custody. Unfortunately, Mr. Hart was plinking out in his yard and the deputies entered his house after putting him in the car.....
...then "Oh s#!t! We entered his house without a warrant! We need to come up with some "exigent circumstances"!
They can plainly be heard discussing "scenarios"!
"Uh...maybe there's some shooting victims downstairs?"
"Don't worry. I'll use flowery language when I write this up."
If this isn't a clear case of the government "manufacturing" its own exigent circumstances, I don't know what is.