In _King Leopold's Ghost_ by Adam Hochschild, the author states:

Although it is painted with too broad a brush and is far from Twain's
best work, "King Leopold's Soliloquy" provoked the royal propaganda
machine to rush out an anonymous forty-seven-page pamphlet, "An
Answer to Mark Twain." (p. 242).

The "royal propaganda machine" is unidentified by Hochschild but
I would hazard a guess it may have included Leopold himself.

Barb