It turns out that like Zachary's discovery, the item was an unfortunate accident. Slaczka explained to Joystiq that "sambo" is used in the game as an alternate term for "fig leaf gourd," an ingredient in the Ecuadorian dish fanesca. "Sambo" is the local term for the gourd. As for the watermelon-like appearance? "We reuse art," he said. "Fig leaf gourd looks a lot like a watermelon. It's just an alternative name in a giant list of tens of thousands of names."
Slaczka noted, offering evidence to the contrary of calls of racism, that many of the human characters in the game, including the fireman, winemaker, and dancer, manifest as black, as does "Brandon," the in-game representation of a 5TH Cell "wordsmith" (one of the employees tasked with finding words for the database), who is of course black in real life. He also mentioned Isaiah, a major character in the developer's last game Lock's Quest.
We must admit, a random, potentially slightly embarrassing art/word pairing in the game seems a lot more likely to us than endemic racism expressed through antiquated epithets. Slaczka told us he didn't even know "sambo" (the slur, not the gourd) until tonight.