The new study refutes earlier findings by researchers at the University of California, Los Angeles, who concluded that the odds of getting head and neck cancers rose in tandem with the frequency and duration of marijuana use.
Marijuana definitely causes cancer
This statement fits into the neither category, because while we know that increased marijuana use increase the risk of cancer, we do not know if this is a correlation or causal relationship. The system may have interpreted this as incorrect because of the indefinite language in the context. 
( z1 cause :ARG0 ( z2 marijuana ) :ARG1 ( z3 cancer ) :mod ( z4 definite ) )