They made a mistake. There are 2,315 relatively familiar words that can be the secret word, and 12,972 words in all that are accepted as guesses.

Using the full list as potential guess words, TARES is optimal for a random secret word from the full list, and SOARE is optimal for a random secret word from the short list.

Neither of these words appear in the short list. If one sticks to the short list for both guesses and secrets, RAISE is optimal.

The game has two lists -- the guessable words list, and there's also a subset which is the possible answers list (this is also provided to the players, I think maybe in order, even -- it is just for fun after all, no need to prevent cheating). IMO it is better to stick to the guessable words list.