Said letter proves that they actually *didn’t* say it transmitted from human to human?

Not true. Any healthcare professional, e.g. the WHO, would be able to tell from that email there is human to human transmission. The Taiwan health minister went as far to state that the WHO are "professionals acting like amateurs" due to the very reason that the WHO's response was akin to a lay person reading the email.

Here's the email:

“News resources today indicate that at least seven atypical pneumonia cases were reported in Wuhan, CHINA. Their health authorities replied to the media that the cases were believed not SARS; however, the samples are still under examination, and cases have been isolated for treatment,” the body of the message said.

"Atypical Pneumonia" indicates that this is likely a new disease and not one of the usual, such as A. pneumoniae or H. Hemophilus.

"Not SARS" indicates that this new disease is very similar to SARS, but they ruled out SARS. This alone indicates that there is human to human to transmission, since SARS has H to H transmission and this new disease was similar enough that they felt the need to test if it is SARS.

Finally, "cases have been isolated for treatment" further shows that there is H to H since strong action was taken to isolate the patients in order to prevent spread.

The wording of this email is meant for professionals, who the WHO supposedly are. In medicine, you often have to strongly imply rather than outright state, especially when you have a shortage of facts. Taiwan had limited resources when investigating, so based on what they observed, they provided a very strong hint in medical jargon that there is H to H. Source: work in healthcare.