Earlier quoted context omitted.
You’re probably aware that no fund manager would accept your offer. But it doesn’t prove that they don’t think they can beat the market (as misguided as that belief might be), it just means they’re not willing to take on an absurd amount of risk to prove it.
I don't think the risk is "absurd". Or, at least it's no different than the risk they ask any investor to take by charging them 1% of their portfolio for it to be "actively managed". Plus, they are being compensated. I'm offering 90% of the returns above the index :-)
Of course, if you offered your bet to all comers and gave significant publicity, unknown "funds managers" would be happy to take you up, though they might well default if they lost.