This is a bizarre attempt to translate a modern English song to "ancient French" (or better old French) and belongs to a new musical trend named bardcore.

The wording is relatively accurate although for example I doubt saül was written with a diaeresis. Saul was a way to write the modern saoul. Grammar is far too close to modern French.

The pronunciation sounds very odd.

While we obviously have no recordings about how ancient French was pronounced and it is true letters were seldom mute at that time, we have a lot of clues about the pronunciation (see http://lespascals.org/docs/MethodeAncienFrancais.pdf) and there is no reason for old French to sound like written French pronounced with a strong English accent…
In particular, the stress which has in most cases stayed identical in French often sounds misplaced.

To Jiliagre’s answer, I’ll add that these lyrics are completely ignoring how Old French (unlike Modern English or French) had a case system! There were two cases, and nouns in the subject case (Cas sujet) often looked like modern plurals: in un joueur requiert, that should be uns. Most Modern French words are derived from the oblique case (cas régime) forms of the nouns.

Definitely not old French. Appart from the old fashioned orthographe, you could definitely read it in 19th French littérature.

The singer (by the way, strong American / Canadian accent) simply pronounce last consonants sounds as old French putatively did (ongoing debates about this).

The scientific analysis is in the other answers (especially regarding your question on the last line)

Now, about the title: if you want to know how much this text and song are understandable to a French, I did three passes (native French speaker, amateurishly interested in the Middle-Ages).

First, knowing the English contemporary song and therefore knowing what to expect I listened to it without watching, mentally transalting the lyrics from English to french, and comparing to what I hear.
Success rate: 1 out of 10 words matched

Then I listened while reading the text in French and again trying the translation part. This was a bit too much for me.
Success rate: 3 out of 10 words matched

Finally, I just read the text, loosely listening to the song.
Success rate: a general understanding of what is going on in the song, the words closely matching contemporary words were fine (including some conversions such as es → ê and similar). Specific words which ether have no meaning if contemporary French, or a different one were skipped.

Conclusion: if I was to speak with someone though that "language" I would have major issues to understand the basic meaning of what they want to convey. I would probably die.

Seems very close to modern French to my ears, but it’s been performed for its entertainment value, not academic value. There’s another version of the song here.