None of us know the specific language in the Emmis-Alexander contract, although what Emmis states in the press release Steve reposted from their website sounds like standard boilerplate for a contract covering high-rated air talent.
But ... Alexander is also represented by counsel, and certainly his agent would have a contract reviewed by an attorney specializing in entertainment law before it was signed, so we must presume the Emmis-Alexander contract was so reviewed before it was signed.
If his legal counsel advised him that he was clear to take the iHeart offer, then either there is some language in there that will likely come out during the trial or that attorney should be sued for malpractice. If he was advised to stay at Emmis and he jumped anyway, then he is an idiot. Since I do not believe that he is an idiot or that he was advised by incompetent counsel, there must be something we do not yet know.
Emmis obviously thinks they have a case. It may well turn on language which is subject to interpretation.