If someone is consistently threatening violence at some point you need to make the determination about how to deal with that. You can of course do nothing about it but if the person eventually goes ahead and commits violence you now have a bunch of injured or dead people, which seems a worse ill to me than deplatforming.
Don't deplatform him. Investigate him. Get a warrant and find out who's listening to him. Find out what's going on in their private communications - are they making actual plans, or are they just talking?
Deplatforming is about the laziest and least useful possible response. As Fins said, deal with the violence (including plans to commit it).
To go back to the original article, though, it is interesting that they are banning white nationalist content. Does it mean that any other nationalist content is just fine. And can I start an Even[0] separatist movement on FB, calling for eradication of the whitey? We'll start with Russians, of course, but after them we're coming for you! /s
If I didn't have better things to do, a conspiracy theory of FB (they did elect Trump, as everybody clearly knows) being in cahoots with Stormfront and specifically calling out whites to stir resentment almost writes itself here.