Translation:I said it wrong.
Good question. I guess here the whole of 说错 is considered as a verb as for example 看病 for see a doctor. Same with 吃饭, I may be wrong but it seams to me you can either say he already had (lunch, dinner, ...) 他吃饭了 or he already ate cooked rice 他 吃了饭. Or is it just in Cantonese ?