Chris Forsberg states that Kevin Durant is in a different stratosphere to where Jaylen Brown is. I think everyone outside of Celtics fans would agree that Durant is currently a couple levels higher than Jaylen, but "stratosphere" seems a bit hyperbolic.
Besides the huge age (and salary gap), Forsberg rightfully brings up that the Nets would demand more than just Jaylen. They'd want Robert Williams and picks as well. If you look what they got for Harden, you can only imagine what they'd want for Durant.
The point is, it's cute for Celtics fans to tell Kevin Durant on social media that he should come to Boston, but he's not a free agent.