I think it's because of the sentence structure. This type of Hawaiian sentence is made up of two parts: [a tasty poke] and [this]. The second part is the subject, so in this case, "This" is the subject of the sentence. The two parts get switched around in English to [This] (is) [tasty poke]! Even though the meaning is basically the same, I think the course is trying to teach the Hawaiian sentence structure :D

(Someone in another thread compared the word-order to Yoda-speak, i.e. "A tasty poke, this is!") :D