How is COVID-19 transmission tracked with any accuracy?

Question

I saw this article: New study finds few cases of outdoor transmission of coronavirus in China (and study it is based on). I don't have a science or medical background so maybe this is obvious but how can they draw such conclusions when:

1. COVID-19 can have a long incubation period (e.g. "14 days" as has been reported).
2. An infected individual may be asymptomatic.

How can they state with any surety where an outbreak occurred (e.g "a transport setting", "outdoor environment", etc)? Are they tabulating, say, all gardeners who ride a bus to work with those who walk, drive, bike, and then with rates of infection?

What am I missing here? (no doubt quite a lot)

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From the article:

Nearly 80 percent of all of the outbreaks occurred in a home setting, while 34 percent came from a transport setting. Additionally, most of the home outbreaks resulted in three to five cases.

However, researchers were only able to find one outbreak that took place in an outdoor environment, involving just two cases.

"All identified outbreaks of three or more cases occurred in an indoor environment, which confirms that sharing indoor space is a major [COVID-19] infection risk," the researchers concluded in the study's abstract.

Answer 1

If you read the actual paper they explain their methodology. It's a review of data published on websites by health authorities and only using data where the information regarding the spaces in which contact tracing identified how a person developed the illness. So it's missing data from any of the large cities because that data doesn't exist online. And it relies on how accurate the contact tracing was by the various authorities.

So it's useful but not definitive.

https://www.medrxiv.org/content/10.1101/2020.04.04.20053058v1