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.
( z0 topic :domain ( z1 sport ) :mod ( z2 main ) :topic-of ( z3 article :mod ( z4 this ) ) )
(z0 / topic
    :domain (z1 / sport)
    :mod (z2 / main)
    :topic-of (z3 / article
                  :mod (z4 / this)))
The main topic of this article is sports .
z0 :mod z2
z3 :mod z4
z0 :topic-of z3
z0 :domain z1