Verstappen took a record 14th win of the season in Mexico City on Sunday and has now scored more points in a single campaign (416) than anyone ever, breaking Lewis Hamilton's 2019 record of 413.
Even if seasons now have more races, with changed scoring systems, fastest laps, and sprints adding to the points available, Verstappen and Red Bull have undeniably raised the bar this year.
The team have won the last nine races and 16 of 20, their greatest haul yet in one championship, and secured both titles for the first time since 2013.
There are still two more races remaining to push the boundaries even further.
"We are actually witnessing something very special. And I sometimes think that his achievements perhaps don't receive the plaudits that they should," Horner told reporters after the race at the Autodromo Hermanos Rodriguez.
"I think actually what we've witnessed this year is an absolutely outstanding performance from a driver who is very much at the top of his game."
There is no question Verstappen gets kudos at home in the Netherlands, just as teammate Sergio Perez does in Mexico, but the sport has become increasingly polarised as it grows in popularity with new audiences.
Social media, as Verstappen observed on Sunday night, has become "a very toxic place" populated by "keyboard warriors" far removed from the racetrack.
"He's won the most grands prix in a year now, within 20 races, on top of that he's won two sprint races and he's not won all of them from pole position," Horner observed.
"He's had to fight and race for a lot of those victories. When we look back at the end of the year, it is an absolutely outstanding year that Max has driven. He hasn't put a wheel wrong, he's been perfect throughout the season.
"It's been incredible, the level of consistency that he has been able to achieve."
Verstappen's first title in 2021 came amid controversy in the season-ending race, with a late change in the safety car procedure allowing him to overhaul Hamilton on the last lap for victory.
There has been no such suspense this year, with Verstappen wrapping up the title with four races to spare, but the team's breach of the budget cap last year has still allowed some to question their achievement.
That clearly rankles, with Verstappen and Horner showing their sensitivity by refusing to talk to Sky television over the Mexico weekend after a pit lane reporter referred to Hamilton being 'robbed' last year.
"Accusations of championships being robbed is something we don't feel is an impartial commentary ... Max is very upset about it and as a team we support him fully," said Horner.
