An island grammar is a grammar that only describes a small chunk of the underlying language. It is used in language parsing in situations where there is no requirement for checking the entire syntax of a provided text. Island grammars can be extended with the use of a bridge grammar.
The main topic of this article is sports.
It's definitely incorrect because the main subject is language/grammar. It's difficult because verifying the statement requires a high level understanding of the entire text, and it might not know what "main topic" means exactly.
( z1 topic :mod ( z2 main ) :poss ( z3 article :mod ( z4 this ) ) :domain ( z5 sport ) )