Why is 'He told the truth in the interview' marked as wrong? The answer suggested is 'He said the truth...' which is not normal English usage.
Yes, but some phrases can be formed with only one of the two, i.e. tell the truth, tell a story, tell the difference, say something, say a prayer etc.
At the risk of splitting hairs: I would say 'told the truth' is more often used in a situation where there is a challenge and one confesses, whereas 'spoke the truth' is more neutral viz Public servants should speak the truth to the powerful. In the case here I think either should be accepted.