I've tried the following code:
simplex.goto("github.com/mitchellh")
num_contribs = simplex.extract_text("blabla")
print(num_contribs)
It outputs all texts from the page. Is it expected? Maybe it should fail indicating element could not be found? extract_text returns all the lines of text it finds within an element. Looks like for your case it selected a majority of the page from the description, so it's returning most of the text on the page.
We currently don't return failure cases (just closest match) -- but good suggestion! We'll fine tune on some negative cases and see if we can catch them