Comment by thesz

> there is no probabilistic link between the words of a text and the gist of the content

Using n-gram/skip-gram model over the long text you can predict probabilities of word pairs and/or word triples (effectively collocations [1]) in the summary.

[1] https://en.wikipedia.org/wiki/Collocation

Then, by using (beam search and) an n-gram/skip-gram model of summaries, you can generate the text of a summary, guided by preference of the words pairs/triples predicted by the first step.