You guys should really stop thinking about it like "this guy is or isn't plugged in".
This dude works in the studio at 98.5, the radio station most used by Celtic brass for interviews and such. He's not a reporter. Him being right on the Horford thing doesn't mean he's "an insider". Just like being wrong on the Sox means he's not.
People can have connections with FO members. People can hear things from those connections that they might tell other people.
He doesn't have to be lying or telling the truth, I think he honestly is just telling people what he heard from someone close to or in the FO. He doesn't have to be vindicated by a trade happening. I don't are why it's improbable he's just saying what he heard?