Here's Thomas North's translation of Plutarch's work:
"...she disdained to set forward otherwise but to take her barge in the River Cydnus, the poop whereof was of gold, the sails of purple, and the oars of silver, which kept stroke in rowing after the sound of the music of flutes, howboys, cithernes, viols, and such other instruments as they played up in the barge.
Here's Shakespeare's work:
"The barge she sat in like a burnished throne, Burned on the water_ the poop was beaten gold; Purple the sails and so perfumed that The winds were lovesick with them; the oars were silver, Which to the tune of flutes kept stroke, and made The water which the beat to follow faster, As amorous of their strokes"
This is definitely no coincidence. What's more interesting is that even though it's almost a fact that Shakespeare had Plutarch open while writing this, we can actually see him doing some creative work. He added some alliteration, and for some reason we humans love to listen to a series of words that all start with the same sound.
> “The thing about this is that it’s definitely half stolen. There is no possible way that Shakespeare didn’t have North open on his desk when he was writing. But also, Shakespeare made little changes. That means that we can actually watch Shakespeare working. We can peep back 400 years and see the greatest genius who ever lived scribbling away. We can see how he did it, and it’s really pretty bloody simple. All he did was add some alliteration.”
Excerpt From: Forsyth, Mark. “The Elements of Eloquence: How to Turn the Perfect English Phrase.”
No that's not all he did, he also included some striking images, the throne, the perfumed sail, the lovesick wind ...
Is it possible that what happened was that pre-digitization research was too time-consuming, so the OED overemphasized easily available sources, like Shakespeare, when looking for first uses of words?
But, it's a narrow interpretation of the results that this is novel. Etymologists have always been extremely skittish of assigning firm origins to words, much less calling Shakespeare an inventor of them himself. His demonstrated vocabulary is simply massive, and statistically some of the words are going to be early.
I suspect Shakespeare gets the nod so frequently because he wrote a lot of stuff in Early Modern English that survived and was widely available in the 19th century when work on the OED began. Other writers' works were extant, but not widely read or available. The OED notes the earliest printed use of the word meaning, so even if perhaps some dullard like Algernon Sidney used a word first, almost no one actually reads those works, nor read them in the 19th century.
Hopefully the digitisation of additional early modern and medieval works helps with etymology - always frustrating to search for something and get six competing theories, all of which seem equally specious.
Impelled to try an epigram,
I never seek to take the credit;
We all assume that Oscar said it.
-- Dorothy Parker
("Oscar" being Oscar Wilde)