I forced myself to read twenty pages of this horrendous mish-mash of Celtic, Viking and Arthurian myth, in which all characters speak the fantasy novel dialect of English yet none of them does anything interesting, and then I gave up.
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but is similar enough to strike readers as being Early Modern English done wrong
This is the nub of the issue, isn’t it? How it strikes the readers. From reading the review (which I loved, btw), it looks like the author was throwing in archaic *sounding* phrasing to highlight the fact that the speaker is from ye olden times*. The vast majority of his readers are unlikely to know or care that the grammar is inauthentic.
On the other hand, if he were to use genuinely authentic Early Modern English, the same readers who have trouble understanding Shakespeare would struggle with the dialog.
So, why use archaisms at all then?
Well, as any linguist will tell you, failing to use archaisms of some sort for a character that has been isolated for hundreds of years, would also be wrong. Unless magic is involved, of course.
*probably wrong 😉