According to European surveillance data [1], diarrhea is also a very rare symptom. The authors don't have a convincing argument why this should be a good indicator for COVID-19.
And even then you have to assume that COVID-19 could be the only cause for an uptick in diarrhea. Not impossible, but extremely unlikely.
Judging from my discussions with my MD and peer-group, I'm under the impression that in Germany, at least in my region, virii with that symptom are circulating every few years. Would expect something similar for China.
[1] bottom row in this picture (I couldn't find an English source): https://www.rki.de/SharedDocs/Bilder/InfAZ/neuartiges_Corona...