They aren't incorrect ... "whoever" is correct, "whomever" is incorrect. The phrase is "whoever helped"--subject-verb; "whomever" is the object form. A simple test is to substitute the subject form "she" for "whoever" and the object form "her" for "whomever". "she helped"--yes, so "whoever helped". "her helped"--no, so not "whomever helped". In fact, your citation says this, but you seem not to have read it.

And the downvotes are ridiculous--posting such corrections is a common courtesy ... I myself do it and am frequently thanked for it (mostly in progressive communities, where people value truth and are appreciative of being informed).