I think Hernandez summed it up well, in line with your thoughts above:
"It was quick thinking on Conforto's part to try, but he doesn't know that the umpire was gonna ring him up," Hernandez explained. "He made his decision before. It's just something you don't do. You don't do that. ... It's not cheating. He made that decision at the last split second. It's not cheating.
"It's not like someone goes out there and has a razor blade and starts cutting the ball. Or you got a guy out there that has binoculars in the scoreboard that's relaying the signs or have trash cans in the dugout. It's not that. It was a split reaction and Conforto got away with it. The Mets got away with it."
A split reaction, not premeditated, and without knowing whether it was a ball or strike. He’s gonna get hit regardless, though.