I noticed one of the cited bluesky posts was all in French, so one might argue that technically it didn't find the English word "mouch", but rather a different French word that happens to be spelled the same. But trying to sort that out seems unrealistically challenging. "Mouch" is only in the dictionary as an alternative spelling to mooch, so probably a pretty rare word to see in English.
Bluesky lets you select the language your post is written in before posting it and it is attached as metadata to the skeet. I guess the backend for this only searches posts in English, but it's possible the dataset is not 100% accurate due to some users forgetting to switch language before posting.
