This tweet is a stupid comment. He's clearly not going with the handoff fake, he's looking at the QB the whole time.
What he's doing is playing the pass. Gronk ran a pattern over the middle, so Smith was moving underneath that route, and Godwin was out in the flat to Brady's right, and with Brady first opening up facing his right, either was an option. As soon as Brady turned back to the left Smith switched directions
Notice how the guy who tweeted told everyone to focus on the :43 mark - the close up view that doesn't show everything going on around the field. That doesn't tell the story.
If you look at the zoomed out views you can see that there were other responsibilities and that Smith did nothing wrong. He couldn't know the ball wasn't going to Gronk over the middle or Godwin in the flat until Brady turned back to his left to make the throw he did.