Carlos Sainz won the Singapore Grand Prix with an intelligent drive, heading Lando Norris and Lewis Hamilton to the chequered flag.
It marked the first time the Dutchman hasn't won a race since the Azerbaijan Grand Prix in April, and the first time Red Bull has been defeated since the Sao Paulo Grand Prix – the penultimate round of the 2022 season.
“To win 15 in a row is insanity when you consider the variance of circuits that we've come across the conditions, etc” said Horner.
“It was the first time since I think 2018 that we weren't in Q3 [on Saturday], so it's been a hell of a run for the team.
“We always knew it was going to come to a stop at some point.
“It's been an incredible year, one we're all very proud of and hopefully we can we can be back up at the sharp end next weekend.”
Ferrari arrived in Singapore and enjoyed the fastest car throughout practice.
Sainz capitalised on that by claiming pole position before racing into the lead – a position he would never cede throughout the 62-lap encounter.
That included the use of canny tactics in the latter stages where he backed Norris into the closing Mercedes to create a DRS train.
It was an impressive performance, one Horner was quick to acknowledge.
“I have to congratulate Ferrari and particularly Carlos, who drove a very strong race today and deserve to win the race,” Horner said.
“At the same time, we've narrowed in on both championships, which we have a chance of winning the constructors in Japan.”