It is merely a translation that is not to be taken literally and one that requires context. The passage in that page is from Bernardino de Sahagún’s Florentine Codex, where he lists nahuatl culinary terms and some ingredients sold at markets such as the pre-Hispanic Tlaltelolco here. In this case, “bird excrement sauce” is the literal translation from Nahuatl totocuitlatl molli. Although totocuitlatl is “bird excrement” in Nahuatl, it was also another name for the chiltepin pepper (Capsicum annuum var. glabriusculum). Specifically, these names are briefly described to designate a “very small spice” in the Crónica Mexicana, which discusses the history of the Mexica (Aztec would rather be an anachronistic term here). It is likely that this chili pepper’s small size and roundness, reminiscent to that of fecal sacs, is what earned it its name. To compare, there is a similar case with the plant Ornithogalum umbellatum, whose edible bulbs were referred to as “dove’s dung,” due to its resemblance to the excrement itself, according to Fernald in Edible Wild Plants of Eastern North America.

About the chiltepin pepper, this common name also pertains to nahuatl chiltecpin, or roughly “chili pepper flea,” due to its tiny size and high spice content being comparable to that of a flea bite. Unfortunately, nahuatl terms and names are often mistranslated or require context, such as the commonly accepted etymology for the avocado.