I was unable to find out the rules here and what is the difference:
As yet, he has not received the package.
As yet little was known of the causes of the disease.
In the first, as yet means ‘up until this time’. In the second, it means ‘up until that time’.
That means that in the two example sentences given:
it is the negative triggers (respectively, not and little) that license the use of yet.
We can easily see this from the fact that affirmative variations are ungrammatical:
Executive Summary: What a word, especially a little word, is supposed to mean has little to do with its syntax. Dictionaries are for meanings and pronunciation; grammars are for grammar.