To be fair, the reason why only 90% of English speakers would get every word in that poem right is because there's a lot of very uncommon words in there. 'Sward' or 'phlegmatic' are not things I've ever heard used in casual conversation. I'm pretty well read, but I have never heard of Terpsichore before today. Similarly, Arkansas is an American peculiarism - I wouldn't expect a non-American to get that one right, same with Islington being a UK peculiarism. Probably should have left names out of the poem, because names have a ridiculously wide allowance for pronunciation.
It's also interesting that the poem writer has a particular accent:
Rounded, wounded, grieve and sieve,
Friend and fiend, alive and live.
Where I live, grieve and sieve rhyme, and neither rhyme with any verson of 'live' (though they do rhyme with 'leave'...). To me, sieve is 'seev', to the author, it's 'siv'.
The poem is very, very well put-together, but I think that the uncommon words and names should have been left out - the rest of the words (most of them) are pretty common, and most native English speakers that occasionally pick up a book should be able to get them.
The problem is that lack of diacritics - I was travelling through Vietnam and found it funny that they used so many diacritics where English needed none... then I realised English definitely needs them - for example, how do you pronounce 'wind'? It depends on context. If you're talking about air movement, it's pronounced differently to rolling something up on a spool...