This is because he is only extracting the noun phrases from the sentence. If you adapted his code to tag verb phrases as well (by modifying the semi-CFG and the normalize_tags method) then you could also extract "discovering" and "curating" as well.
But this would miss the "main topics," since when you have both the vp's and the np's you have everything :/ Here is the resulting tree (it's unformatted, sorry, I tweaked an old Prolog grammar I had for analysing search keywords and tweets):
