The Ninth Circuit is exactly right here. So right that it doesn't need oral argument, nor more than a week after the date the case was deemed submitted to publish its opinion.
It may well be (and in fact is the case) that, in California state court, when the trial judge grants an anti-SLAPP motion that dismisses the claims against two of three defendants, that order is subject to an immediate appeal. But, under the Erie doctrine, the same rule doesn't apply in federal court. Rule 54(b) expressly says that such orders in federal court are not "final" orders and hence are not subject to immediate appeal. You've got to wait until the end of the case. That rule is dispositive here.
Technically, the Ninth Circuit's per curiam opinion leaves out one (important) step. The opinion says that because there's a federal rule of civil procedure that governs the issue, federal courts follow that rule. But in truth, the Ninth Circuit would also have to conclude, under Erie, that this rule is also arguably procedural and does not abridge, modify, or enlarge any substantive rights. That's the test.
I'm certain that that's what the panel was implicitly thinking anyway. Though, again, technically, they might have wanted to actually say it.
Still, the result is definitely right. Appeal dismissed for lack of jurisdiction.