(You know which meaning applies in the two example sentences, but I don't think it does any harm to spell it out.)What do you think about these definitions?
Yet in negative sentences:
(1) It can emphasize actions that have not occurred between an indefinite period of time in the past and now. In short: before now.
(2.a) It can show surprise that actions have not occurred by now.
Fine, but you need to look at the underlined bit.
Is it better this one? 'Is this one better?'
(2.b) It can show surprise that actions have not occurred at some time before now. I don't think so.
The tone of voice is also very important to distinguish which definition somebody is referring to. But people aren't referring to meanings.
(1) Kid: I haven't done my homework yet.[-> 1]
(2) Mother: You haven't done your homework yet! THAT's incredible! Two hours for one page![->2]