Red Bull has dominated the season so far, with Max Verstappen and Sergio Perez sharing in four one-two finishes, and only missing out on another in Australia, where problems for the Mexican in qualifying saw him recover to fifth at the flag.
Horner says that Red Bull made the progress "that you would expect" from its 2022 model to this year's car, while rivals – essentially traditional frontrunners Ferrari and Mercedes – have failed to keep up.
"You can never write anybody off," Horner said. "And I think it's been the best start that we've had.
"We feel that we've made a good step from RB18 into 19, but the kind of step that you would expect, I think it's more that it feels like others have lost ground.
"I'm sure that they're working hard to address that. So big gains could come quite quickly."
However, Horner cautioned that rivals could still catch up, with the next race in Imola seeing a round of upgrades from most teams.
"I think it's surprised us that the others have perhaps underdelivered compared to where they were last year," he noted.
"But no doubt they will be looking to address that. I'm sure, starting in Imola, we're going to see big updates starting to come through.
"We've got a great car, we've got a great team, we've got two great drivers. But still a long way to go. Let's reserve judgement until after we see what they turn out with Imola and Barcelona."
Source: Autosport