I agree about "sank" still being the correct past tense for "to sink".
But are these two expressions really equivalent?
When "der Groschen fällt" or in English (BE?) "the penny drops", something that one had not understood suddenly becomes clear.
But when someone says, "It hasn't really sunk in yet", the fact -- a sporting victory, a lottery win, the news of a death, etc. -- is perfectly easy to understand, but it takes some time for the mind to adjust to it and accept it as a reality.
So these seem to me to be two different things.
Even in California's second example in #1, I would take it that the writer had seen plenty of clear evidence but that for a long time his mind had resisted drawing the conclusion that he finally drew.
(In the first example there, the first "it" refers to the tournament, see the preceding sentence in the article.)