There were still two minutes of injury-time left when Down manager James McCartan slipped off his earpiece and walked along the sideline to shake hands with his opposite number.
And, tellingly, while the ground rocked in celebration, Donegal's players reacted the least hysterically and left it to their manager to explain why it meant so much to them.
From party-boys to seasoned pros, Donegal's footballers have undergone a remarkable transformation under McGuinness.
"This is the same group of players that were heavily criticised for a number of years," he said.
"They've come round, won Ulster titles back-to-back, and a lot of players that were at the centre of that storm, criticised for their hunger and their application for Donegal, they're the boys leading the line now."
The final scoreline, however, doesn't reflect the fact that the Mourne men threw down a very impressive first-half challenge. But it was the ruthless manner in which Donegal then smothered them into submission and hit the turbo button that was most significant.
A team criticised for being ultra-defensive, they had 11 scorers, and impressively 2-15 came from play.
They also, thanks to a great Paul Durcan late save, maintained their record of conceding just two championship goals (both from penalties) in the last two summers.
Down burst out of the traps playing with the sort of tenacity and energy that has become their opponents' trademark. They looked like they were deliberately chasing goals and certainly rued missing three of them by half-time.
But then Donegal nabbed a goal against the run of play, scored by Leo McLoone. That goal vitally allowed them draw breath.
It helped them turn ends leading by a point (1-5 to 0-7) and you couldn't help wondering if Down had already shot all their bullets.
The match turned on one incident when super-sub Martin McElhinney forced a turnover from Kevin McKernan. In one quick counter-attacking move he got the ball up to Murphy for his first (and only) score of the game and when Mark McHugh added a point from the next play, Donegal's blood was up and Down's immediately sank.
Donegal turned the screw and their second goal came on 53 minutes from Frankie McGlynn, who was marvellous throughout.
Not even Coulter's reintroduction could rouse Down, who faded badly.