There are legal and logistical challenges in preventing US residents from entering or imposing quarantine.
Would strong or (more importantly) earlier responses have been good? Absolutely! But a weak and late response is much better than an even later response (even if the later response is stronger).
By the numbers we should prefer a non-US-resident ban-- even without screening/quarantine-- on day 20 than a everyone-ban on day 40.
He was even called a dictator for that.
Correction: South Korea & Japan were "do not travel" not specific travel bans.
Japan: level 2 (exercise caution) https://travel.state.gov/content/travel/en/traveladvisories/...
Korea:level 3 (reconsider) https://travel.state.gov/content/travel/en/traveladvisories/...
Regardless, they're still a response which refutes your original point.