I do get that argument; I just don't think it holds any water.

If one hires a hitman to kill someone, that one may still be held accountable to manslaughter, even if that specific person didn't kill anyone themselves. It may also not matter how many degrees of separation are there between that person and the hitman: as much as putting a (Bitcoin) bounty on someone's head (with a "smart contract" or whatever) may be considered manslaughter, even if nobody knows who the actual hitman is.

"Agency" is not a magic get-out-of-jail card.

Also, one may try convincing a judge "Your honor, it's true that I wrote the code that encrypted the plaintiff's network and wrecked a havoc, but I did NOT execute it; the plaintiff could have instructed his CPUs to not execute my code"; I don't know if this argument would hold.

Finally, there might be a "reasonable burden" argument in this case. It's reasonable to expect that website builders would know how browsers/internet work. It's not reasonable to expect that website visitors (general populace) would know that. Hence, the burden of GPDR compliance is better put on the builders' shoulders, which is exactly what happened in this particular case.

Summary: A company did try the “it was the browser, not us” argument in the “Fashion ID” case. The court did not fall for it. Data controller and thus responsible for compliance is whoever determines the purposes and means of processing. Being able to control what the website does seems to be good evidence for being a data controller.

In this Google Fonts case, the website operator didn't even try this discredited argument.

   > Sure your honor, the victim died by carbon monoxide asphyxiation, but it was his choice to inhale the gas, even though it smells the same as normal air"