I like Weiss, but I've never heard him break any kind of news. Horford might be traded for a myriad of reasons, he might not be. Pretty much the only players Brad has said aren't being traded are the J's, so sure, I imagine the Celtics have had a phone call, but I don't think this is much of a scoop as opposed to saying something obvious, can't be disproved when it doesn't happen, and makes you look good when it does.
I don't think Horford will be traded right now. He's not getting salary dumped, so he's only moving for one of two reasons:
1) There's a contending team out there with $20+ million of salary they want to move and a need for a big.
2) There's a high-level or even star player the Celtics need to use Horford's salary to match.
I can't think of a team that fits #1. #2 is probably not happening because those deals tend to be rarer. But sure, the Celtics have made calls.