The whole "The_Donald was a FBI honeypot" is a pretty strongly persistent urban legend. And obviously false. Surprises me how many people thought that was legitimate.
Ask yourself: Shouldn't there have been results sooner? Would FBI (or whoever) have left an obviously dangerous forum open for honeypot purposes for years and years? If this really was a honeypot operation, wouldn't they have made arrests as soon as they could? And if this really was the case, shouldn't we have the details by now (FOIA requests or whatever)?
Of course not. US law enforcement would have immediately involved themselves if there were any credible threats posted there. But they don't really do much about festering pits of far-right radicalisation. It was left open with the full approval of Reddit admins. Can't really claim ignorance on their part either.