In my opinion, the quarantine just worked.
Here's my thinking. We know the following (maybe my information is not entirely correct, but the reasoning is still valid):
1) a host is infectious for about 14d;
2) the virus survives on surfaces/objects for 9d (this is based on other coronaviruses, but a more recent study suggests it's shorter).
If you want to stop the spread, or reduce it significantly, you need to quarantine people for at least 2 x 14d + 9d.
Why is that? Because, the first infected in each household will stay so for the 14d, and assuming all the other members of the household are infected by the first person, and assuming the infection happens towards the end of the first 14d, for safety, you need another 14d. Plus, you want to make sure the virus is not still active via surfaces/objects, so plus up to 9d.
If you go further and assume that not everyone is infected by the first person to have the virus in the household - so you will have multiple generations of infections - then you need to add maybe another 14d. And so on.
China did some 2 months of quarantine. Chances are very high they actually succeeded in reducing significantly the number of active cases. No need to appeal to conspiracy theories at this point.
Why do we see a resurgence of cases? Because the 14d is just the avg. Some people reportedly can be sick for longer, many can be sick without having serious symptoms, some people don't respect fully the quarantine etc. Because of this, stopping it permanently is going to be very difficult via quarantine alone.