Saints cruised into the Old Trafford showpiece a fortnight ago.
But Leeds did it the hard way and skipper Sinfield claimed the reality of facing elimination helped them refocus and repeat last year's Grand Final upset.
He said: “Two weeks ago we got our bums smacked - but tonight I just thought our desire and commitment and willingness to scramble for each other was evident.
“It got us through last week and to lift a trophy... it’s a special moment.
“Every man and his dog would have had a bet against us tonight.
“We lost Brent Webb, who’s been outstanding for us this year. To not have him here tonight is a shame.
“We’ve got guys in the squad who’ve missed out, who’ve played a part this year - but I thought we had 17 warriors out there.
“Two weeks ago we were at the lowest points of our career, maybe not being here - but thankfully we are here again.”
Leeds coach Brian McClennan added: “We knew we were playing against a very classy side and that’s why we’re so overjoyed.
“We had belief in ourselves and we are very happy. This is a result of the whole year, not just one game."