The problem was on the trust. 20 millions was quite small but if you are seeing the market and these days people are starting to distrust the platforms that have gotten bad news. OP was a new platform and this news already given very big impact to the trust from the community. Trust is everything in the crypto. Your project can't survive without the trust from the community
This point is quite true because for the community to buy and invest in your project you would have to prove how legit and transparent you are.
And for the Optimism project, their recent activities stands to create FUD in the community which is bad for business.
If we take into account that the capitalization of the OP coin is 3.5 billion dollars, then the stolen 20 million are not critical for the coin from a financial point of view. Therefore, the decrease in the value of the coin should not be considered as a consequence of hacking, since the price of the coin is constantly decreasing after the initial pump.
I don't even think we can call it hacking because from what I've read, it says here that the mistake was on the part of the OP team for sending the tokens to the wrong address in the first place.