Yeah for the first time I'm starting to get concerned this stuff is very real now, even knowing Brad Stevens and the organization typically work in silence.
Barry Jackson and Ira Winderman are two of the best Heat reporters out there, and they both are saying Boston has a good shot at landing him and Barry is even replying to people on Twitter saying it's the common belief around the league as well (agents, reporters, etc.).
Then you add in Stein, Fischer, Amick, etc. Stein in particular is also pretty good at this stuff, a lot of stuff he says at the beginning of negotiations/trade talks end up materializing later on.
I'll also add, yes some of the trades like Jrue, Porzingis, Brogdon, etc. happened without much rumors or smoke, but those two still aren't to the level of Giannis and I don't think there was as much interest in those guys as there is Giannis obviously, and with multiple teams involved including the need for a third team to facilitate, it's hard to keep it tight-lipped for long.
Might be saying goodbye to Jaylen soon

Either way though, it's Miami or Boston. Giannis has some control on where he goes especially with the future extension and I doubt teams will trade for him without it (such as Portland, though POR may love Jaylen instead)