Semantic definitions involve the vocabulary of the genre, the sensual trappings. It clues us in to what we're watching. The problem is that it's incredibly flexible and can cross a few wires. I'll explain in a second.
The syntactic definition is the core themes that define the genre. In horror for example, it's commonly argued that the syntactic definition of the genre is, "Normalcy is disturbed and threatened by the Monster. The Monster defies our reality, cannot be reasoned with, and exists only to destroy."
The problem with taking a purist approach with either definition is a lack of nuance. For example, Near Dark has many of the semantic elements of a Western, but it also contains syntactic aspects of a horror movie as well. Outland is a Western by syntactic definitions, but it has the semantic vocabulary of a science fiction film. The examples just go on.
Alien (horror/sci-fi)
The Matrix (wuxia/sci-fi)
Star Wars (samurai/sci-fi)
Shrek (romantic comedy/fantasy)
Princess Mononoke (war/fantasy)
The Dresden Files (noir/fantasy)
Yes, I know I watch/read a lot of sci-fi and fantasy.
The point is, writers very deliberately consider the semantic and syntactic definitions of genres before they commit their ideas to paper. In my experience, most magicians are not only terrible actors, they're terrible writers as well. They have no idea what they're doing, and none of their presentations are ever consistent enough to clearly use a semantic or syntactic definition of anything.
This also means there's a lack of message. The syntactic definition in particular does not stifle diverse messages. It simply provides a framework. For example, take the horror stories of Rosemary's Baby, Dracula, and Alien. While adhering to the syntactic definition they are still able to explore themes of motherhood and the feminine, the repression of sexuality, and the integration of the masculine and feminine to protect the family unit respectively.
Most magicians never think of these things. Ever. They're too busy fantasizing about how many girls they're going to impress. And for magic to be taken seriously as an artform, it needs to start acting like one and take things like this into consideration.