In short: Can you see the shell game there?
Merchan WOULD HAVE ALLOWED HIM to Testify what Smith wanted IF AND ONLY IF the PROSECUTION could have their OWN expert witness.
But the DEFENSE wanted ONLY THEIR expert witness and not a corresponding one for the prosecution.
See the game being played? And now they complain “OH HOW UNFAIR” when they could perfectly well have had him testify those things