I remember the only international flights allowed as being the repatriation flights. (e.g. when an overseas government organised for some non-Chinese who were in Wuhan to be able to come home). And presumably the return flights for planes that would otherwise have been stranded in Wuhan.
FactCheck matches my memory...
https://www.factcheck.org/2020/05/trumps-flawed-china-travel...
Wikipedia for 23rd January says all outbound flights were banned:
https://en.wikipedia.org/wiki/Timeline_of_the_COVID-19_pande...
And anyway, there's something wrong with the whole idea: if domestic flights were banned international flights were still allowed, wouldn't the people of Wuhan who wanted to get to Beijing just fly via Bangkok or Seoul? A ban on domestic-only flights wouldn't have achieved anything.