The vast majority of laypeople (inclusive of people who would be getting their information from the WHO's Twitter account) would readily interpret "no evidence of X" to be equivalent to "we shouldn't be worried about X". To pretend otherwise betrays a misunderstanding of how laypeople think.
If the WHO had a more nuanced stance, they should've made that clear - e.g. "We don't have evidence yet that the #coronavirus spreads human-to-human, but it's possible".
But the thing is, that was their stance. I agree that their tweet might show a lack of scientific communication skills or scientific education of the general public, the official recommendations have been very clear. Countries and public health organisations don't base decisions on a tweet. They go to the official guidance.
"The primary objectives of surveillance are to:
1. Detect confirmed cases/clusters of nCoV infection and
any evidence of amplified or sustained human-to-human
transmission;"
Then on Jan 10th:
https://apps.who.int/iris/bitstream/handle/10665/330374/WHO-...
"As information about the etiology, clinical manifestations and transmission of disease in the cluster of respiratory disease patients identified in Wuhan is limited, WHO continues to monitor developments and will revise these recommendations as necessary."
Neither of those directly say "it's possible that the novel coronavirus, like other coronaviruses, is capable of human-to-human transmission", with the sole exception (to your credit) of one quote buried in a header in that second document: "route of transmission unknown but suspected to be respiratory".
Also, those documents seem to be focused entirely on laboratory testing and surveillance, not on, you know, treatment or prevention (beyond one section on preventing infection among medical/laboratory personnel, whence I pulled that quote) or risk assessment.
If these are the documents used for policymaking, then it seems their lack of communication skills extends even here. They'd benefit from a more bottom-line-up-front approach and making these suspicions more explicit; the documents as they stand read as a wishy-washy "well we don't quite know how this spreads and don't want to jump to any conclusions...", and it should be entirely unsurprising that policymakers interpreted that as "aight, WHO hasn't said it spreads between humans so nothing to worry about".
If the WHO had a more nuanced stance, they should've made that clear - e.g. "We don't have evidence yet that the #coronavirus spreads human-to-human, but it's possible".