Honda announced its F1 exit in 2021 but agreed a deal to continue to supply its power unit for chassis partner Red Bull until the end of 2025 and the demise of the current engine regulations.
But the Japanese manufacturer will return to the series in 2026 as the works partner for Aston Martin, with Red Bull having already created its own in-house Powertrains division in the meantime.
Max Verstappen, who then delivered Honda the 2021 and 2022 drivers’ championships and helped secure last season’s constructors’ crown, described it as "a shame" to see Honda partner Aston.
Horner added that if Red Bull had known Honda was keen to return, it never would have embarked on its own engine programme even if it had admittedly “outgrown” being a "customer".
Asked by Autosport for his reaction, Horner said: “Well it’s certainly an expensive decision!
“For the prospects of Red Bull, we’ve outgrown being a customer.
“For us to have the power unit on-site, integrated fully with the chassis and the synergies that creates with engine and chassis engineers next to each other, for the long-term, the advantages are significant.
“We wouldn’t have made that jump had it not been for Honda’s withdrawal."
Source: Autosport