The Lions crossed the try line seven times, securing consecutive URC victories after opening their campaign with three defeats, and now sit eighth on the standings.
It was also the first defeat of the season for Ulster, who were coming off wins over both the Bulls and Sharks, making the win a particularly rewarding for van Rooyen, who singled out the team's defensive effort late in the first half as a pivotal moment.
"The guys really played hard today. A lot of effort," Van Rooyen said.
"For us to get five [log] points, obviously we're happy with that. But I'm more proud of the work and how we did it. We had unbelievable sets on defence and attack.
"If you take the hard work in that last defensive set before half time, it was inches in the corner. We prevented that try, which showed how hard and desperately we were working."
Despite the impressive performance, van Rooyen stressed that there is still plenty of room for improvement.
"We understand we need to get better and more clinical, but we also understand what pictures we are creating," he added.
"Everyone's emotional after a win like that, but there's still [only] 10 or 11 points on the log.
"The feeling in the camp is very different from what people think. We know we're moving in the right direction."
Daniel Smith's blistering knock helped Western Province overcome the North-West Dragons in their tournament opener at Newlands, while Boland notched their second consecutive win, edging the DP World Lions at the Wanderers.
The Sharks have announced that head coach John Plumtree will step down at the end of the 2025/26 season as part of a broader restructuring of the club's coaching and rugby operations.
Sharks head coach John Plumtree was pleased with his side's United Rugby Championship bonus-point victory over Scarlets, but acknowledged that his team is still a work in progress.
The Stormers, Lions and Sharks all picked up wins in another strong showing by the South African teams in Round 5 of the United Rugby Championship.
