You missed the second part of that section of the act..
A person commits an offence if he-
(a) fails to stop when required to do so under this section,
(b) refuses to answer a question addressed to him under this section, or
(c) fails to answer to the best of his knowledge and ability a question addressed to him under this section.
So a person who is not suspected of committing any offence at all - can be detained and compelled to answer questions. Refusal to stop, refusal to answer questions and failure to answer to the best of your knowledge are all criminal offences!