Well, maybe. I want to be up-front that I haven’t read the actual lawsuit, but it seems from the article that the claim is that youtube and reddit both have an algorithm that helped radicalize him:

YouTube, named with parent companies Alphabet Inc. and Google, is accused of contributing to the gunman’s radicalization and helping him acquire information to plan the attack. Similarly, the lawsuits claim Reddit promoted extreme content and offered a specialized forum relating to tactical gear.

I’d say that case is worth pursuing. It’s long been known that social media companies tune their algorithms to increase engagement, and that pissed off people are more likely to engage. This results in algorithms that output content that makes people angry, by design, and that’s a choice these companies make, not “delivering search results”.