China locked down when they were seeing around 400 new cases per day, with that rate roughly doubling every two days. Your idea that they saw only 500 cases in the amount of time that Italy saw 10,000 is incorrect.
Also, due to incubation rate and symptom onset, rate of detected cases usually takes well over a week to respond to lockdown measures. All indications are that Hubei's lockdown was instantly massively successful at preventing spread, but confirmed cases continued to grow exponentially for over ten days because the infection had already spread so far by the time the lockdown happened. China's current rate of case influx is low because their social distancing measures have been effective; Italy is still seeing the exponential growth from the period before they implemented any serious measures.
