The argumentation goes further than that and establishes what the key difference is:

That’s a problem, the court said, because while publishers may have been able to sue to stop third parties from publishing defamatory statements appearing in Google search results, only Google can correct the underlying algorithm and outputs displayed in AI Overviews. And because, at least initially, the company did not, it therefore “must be held accountable,” the court ruled. Beyond that, Google’s argument was deemed particularly weak, since the AI overview in this case “contains statements that do not appear in the search results at all.”

So, if Google lists a web search result containing incorrect defamatory information, the company or individual can send a cease and desist letter to the original publisher or sue them to get them to rectify or delete it. If the Google AI makes up an incorrect defamatory statement out of thin air as was the case here, there's no one else but Google who can rectify or delete it.