What were they legally required to do after their research uncovered this "causal evidence" in your opinion? Shut down?

Cars get into crashes and kill people everyday, yet I don't think it would be appropriate to sue car manufacturers for this harm, solely under the grounds that they "knew about it and ignored it", even if they conducted a study finding a clear causal link between the existence of cars and roadway deaths.

In general, I think our legal system ought not to punish people or organizations for failing to do things that they have no clear legal mandate to do. This is a general claim that I don't feel I need to know all the details of the case to make, though you're certainly welcome to provide specifics if you think they would change my opinion about this specific situation.

> Do you think that the lawyers for meta, the courts and the plaintiffs would have gotten away with such throw away arguments?

No, because unfortunately our legal system doesn't work the way I'm saying it should. If someone feels you have injured them in some way, they can sue you even if you acted in good faith and broke no laws, and if they can demonstrate harm, even indirect harm, they have a good chance of winning. It makes for a very unstable legal environment.