From the article:
"Given any reasonable construction of the words in his postings, those statements do not constitute a 'true threat,' and they are therefore protected speech under the First Amendment," the majority wrote in a 2-1 decision.
So no, Congress cannot simply write a new law to get around that in the 9th Circuit (~20% of the population) or any Circuit that is likely to rule the same way. The observation that they didn't even charge him under a law that criminalized what he said (and if it did, it would be unconstitutional) was essentially just icing on the cake.