The "Relevant Conduct Provision" of the Federal Sentencing Guidelines allows uncharged but related criminal conduct to be used to determine your sentencing at a standard of the preponderance of the evidence. That means that if you're charged with a bunch of stuff and you plead guilty to one count, all the rest can be used to determine your sentence at a significantly lower standard of evidence.

So while the government didn't have to prove beyond a reasonable doubt that his tweets about funding were material support, those tweets could be used to increase his sentence for the Reza Niknejad thing.