It's the difference between:

- There is no evidence that P

- There is evidence that NOT-P

(without accounting for whether higher order evidence [evidence of evidence] counts as first-order evidence).

"There is no alien life in the universe" vs "There is no clear evidence of alien life in the universe".

Or say I have a jug containing marbles. I tell you there are either ninety-nine yellow marbles and one blue or one hundred yellow marbles. I'll draw 10 marbles and you pick "no blue" or "no clear evidence of blue" as your statement. If you're wrong, you owe $1M to charity. I've drawn 10 yellow. Which statement do you pick?

No evidence of something is not the same as evidence of (not something).

No evidence that Person A is the murderer does not mean there is evidence that he is not the murderer(e.g. alibi). Evidence may turn up later on that he was indeed the murderer.

WHO said that there was no evidence of human to human transmission. They did not say they found evidence that it is not transmissible from human to human. That's a world of difference.