I do not want to refer specifically to the Customs Act, but I can say that they are two different things. Under Section 34, a mandatory requirement falls on the conveyance operator. Section 15 of Bill C-12 passed in the 38th Parliament says that each traveller must answer questions.
I do not know if it answers your question. In fact, there are two mandatory requirements. One must be fulfilled before arrival. It applies mostly to the conveyance operator. He can only base his judgment on what he sees or on his suspicions. However, the passenger has no obligation to report an illness to the driver. He or she might even try to hide it.