If you read the introduction to the book at the HebrewBooks.org link, it says that the publisher (apparently not the translator), Ben Tzion Krinfis was given the material in Arabic sometime between 1935 and 1948 by Rabbi Yaacov Moshe Toledano. Krinfis refers to Rabbi Toledano as the 'Rav HaRoshei of Yaffo (meaning Tel Aviv).
A link to Rabbi Toledano is here: https://en.wikipedia.org/wiki/Ya%27akov_Moshe_Toledano
According to Rabbi Toledano's bio, he was elected to that office in 1942 and held that office until 1958. So the citation from Krinfis seems accurate.
Rabbi Toledano, who lived from 1880 to 1960, was a recognized expert on Rambam and is credited with discovering numerous early manuscripts, including some by Rambam in Arabic, from Spain, the Jewish communities of North Africa and also Damascus in Syria.
According to Krinfis, Rabbi Toledano had begun to publish the Rambam's commentary on Esther but didn't finish it. Rabbi Toledano provided Krinfis with what he had done to that time together with an Arabic printing of the commentary that was published in the year 5520 (1759/60).
In terms of the Rambam himself, his son, Rabbeinu Avraham is recorded as having said in his Maamar Odos Derashos Chazal (found in the introduction to Ein Yaacov) that his father had always intended to write something dealing much more with the correct understanding of Midrash. It would therefore come as no surprise if in fact this text was dealing with such subjects.