The Nation of Islam is also classified as a hate group by the SPLC however they aren’t banned under the same laws that protect the KKK.
NOI members just like the KKK have been prosecuted in the past for many things, however outside of very limited circumstances there isn’t such thing as guilt by association in the US justice system.
It's a 100 organizations with 10-30 members who believe they're the true heirs to the KKK. It's be very similar to trying to arrest the head of the Nazi party. There really isn't one.
So no, this is entirely incorrect.
It wouldn’t matter how many Hackernews members started coordinating bank robberies; RICO wouldn’t magically allow for the rest of the Hackernews user base to be prosecuted.
It is indeed a narrow tool, and yet can be applied to organizations such as the KKK.