That tweet was from ten days before the lockdown, based on evidence gathered prior to that. The first death from the disease was 3 days prior to the WHO tweet and Chinese researchers had only identified the specific virus 6 days prior:

https://www.nytimes.com/2020/01/08/health/china-pneumonia-ou...

That very same day the WHO was recommending handwashing / avoiding close contact with anyone with flu-like symptoms / and avoiding wet markets and close contact with animals (since the largest known source of infection was the market in Wuhan):

https://twitter.com/WHO/status/1216754859292184577

It's hard to lay any blame on the WHO for not taking the virus seriously enough in the first days of the outbreak when major politicians were still dismissing it two months later.