Hi Cor, thanks for joining the Forum.
This is a great question and you are right that this study is relatively unique because they draw on a cohort where they have exisiting data about handwashing and so were able to use this as a baseline. In time I anticipate we may see more studies like this or perhaps case control studies that look retospectively at handwashing as a variable in transmission. Indeed we saw quite a lot of studies like this during the SARS outbreak and have similar studies for Cholera too.
Even though we don't have many studies like this for COVID-19 we can still be confident about the effectiveness of handwashing with soap to destroy and remove SARS-CoV-2 from hands. This is because of how soap works at a micriobiological level. This is explained in this Hygiene Hub resource. 13
I also recommend this database of COVID-19 research15. Its easy to search and can be a useful way of keeping up todate with the latest evidence.
The video below is also a great explainer.