Verstappen's busy driving, PR and engineering schedule at Imola did not stop him from competing in the most competitive field of iRacing's Nurburgring enduro with his Team Redline sim squad.
After taking pole in Italy on Saturday, he took on a three-hour stint on his sim in the evening and tacked on another two hours on Sunday morning to help his BMW GT3 entry win the event.
Team principal Horner said he had no qualms with Verstappen taking his sim racing so seriously on a grand prix weekend.
"He is basically a racing machine, so it is quite often he is testing different set-ups in the evening and this kind of thing, so it is not unusual for him to be doing that," said Horner, after Verstappen beat McLaren's Lando Norris to the Imola win.
"He has won two races today. One in a BMW M3 and one in a Formula 1 car.
"You heard what it meant to him. [For] the pole position yesterday, he really had to dig very deep and the performance again today was a masterclass."
Verstappen was put under intense pressure by Norris in the closing stages of the race, with the Dutchman struggling to keep his hard tyres up to temperature.
"In the first stint on the medium it felt like we got everything under control," Horner assessed.
"After we changed on to the hard tyre, the first half of the stint was absolutely fine but we started to lose temperature in the tyre and with that performance.
"The other thing that made it particularly stressful at the end of the race was that Max had already had three strikes on track limits, so Max couldn’t afford to make a single mistake on the limits.
"He delivered brilliantly, so despite significant pressure he was able to manage it and keep Lando just out of DRS."
Team-mate Sergio Perez couldn't replicate Verstappen's strong performance, moving from a disappointing 11th on the grid to eighth.
But Verstappen finished off a strong turnaround from Red Bull after floundering in Friday practice, which Horner labelled as a "big dive" into why its car balance was so off kilter.
"We did a lot of set-up work on Friday night and into Saturday and managed to move the balance and get the car into a much better window," he explained.
"It was a big dive. That is where you must never underestimate the support that comes from the broader team in the factory, the simulator drivers and credit where credit is due, the essence of the team is being able to respond when things aren’t going well and we managed to turn it around.
"This result on Friday, all our analysis would’ve said no way. But we managed to turn it around and win the race."
