The source of the image (FR-9342, as indicated on the right) appears to be Jehan Wauquelin's Chronique d'Alexandre (1460), depicting the Battle of Gaugamela between Alexander the Great and the Persians. Wauquelin claimed that Alexander "had men of brass built and filled with fire to deal with the elephants":

As soon as Alexander heard what these elephants were like he summoned a band of master craftsmen. He bade them forge and fashion, swiftly and without delay, two or three hundred brass statues in the forms of men, hollow inside. This was done at once.

(Alas, Google Books does not have more on this, and I don't have a copy of the source.)

This kind of thing is of a piece with other medieval legends where automatons of some sort were deployed in a somewhat magical manner, including relating to the Alexander the Great story specifically (because Alexander went to foreign lands, it created a lot of opportunity for creative embellishment). Elly Truitt's wonderful Medieval Robots (University Pennsylvania Press, 2015) does not discuss this particular example, but has many other similar examples, including from medieval accounts of Alexander, e.g., an account from the 12th century of automatons defending a bridge from Alexander's crossing:

At the other end of the bridge there were, cast in metal, two boys made of fine gold. One was tall and thin, the other short and fat; their limbs were well made, their faces pleasant. When the army approached and [the two boys] heard the cries, each seized a hammer to block the passage. Above them, two inscriptions drawn up by a cleric proclaim that they were made by augury to defend the passage.

Truitt's book on medieval automata (in the Christian European context, most were literary creations rather than realities; in some Ancient and Middle Eastern contexts, they were sometimes technical realities of a sort, built with complicated gearwork and/or pneumatics/hydraulics) notes that these served interesting ways of indicating cleverness, exoticness, played with notions of souls, etc., depending on the context. I don't know enough about this particular invocation to explain what it is trying to do, but seeing it as an exotic and clever solution to an exotic threat (elephants) seems like a fairly literal reading.

(Note: I am not a medievalist, and really all of what I know about medieval automata comes from Truitt's book and conversations/talks with her over the years. We were in graduate school together.)