I think a crucial distinction between rap and spoken word/(then into) poetry that this author COMPLETELY eschews is the presence of music; i.e., things like "the beat," rhythms to which rap is spoken, presence of items such as refrains or hooks, which do not translate well onto the page or as poetry because of things such as repetition.
I also DO strongly believe that the presence of these music items can, but does not always enable rap lyrics to be "weaker" than traditional poems or even spoken word, because an addictive hook will allow a listener to forgive lyrical flaws. Not all rap is poetry. But a lot of it, the really good stuff, really IS and it's important that poets recognize it as such. There are such brilliant techniques used in rap especially rhyming - where modern day poets could stand to learn some rhythm and jive!
I think in rap meter is king and lyrical/poetic lines are readily manipulated in order to serve meter and rhyme, which can do great things or can result in bad things.
Eminem says it himself:
This aint a song
(from "On Fire" off of "Recovery")
I think "academic" poetry took the left turn into irrelevance at the exact moment the MFA was invented. Work done to impress ones adviser is not likely to be anything but obtuse and rarefied.
It also limits the class of "real" poets to college types. Middle class kids that could afford to have a romantic major.
Of course there are exceptions and in my opinion these exceptions tend to be stronger poets.
Replace poet with painter, journalist, writer or jazz musician and I think it reads as true.