Why did Bedevere mistake "ni" for "noo"?

Monty Python’s ‘Holy Grail’ has a scene (watch on YouTube) where Arthur and Bedevere try to say ‘ni’. Unfortunately, Bedevere can’t quite manage it:

ARTHUR: Ni!
BEDEVERE: Noo! Noo!
ARTHUR: No, no, no, no — it’s not that, it’s ‘ni!’.
BEDEVERE: Noo!
ARTHUR: No, no — ‘ni!’. You’re not doing it properly.
BEDEVERE: Noo! Ni!
ARTHUR: That’s it, that’s it, you’ve got it.
ARTHUR and BEDEVERE: Ni! Ni!
_{Monty Python Holy Grail script 3, Scene 18 http://www.montypython.net/grailmm3.php}

In the Wikipedia article, there is a reference that this is a "a very learned joke about the difficulties of pronouncing Middle English". You can see the reference in the preview from taylorfrancis.com:

Indeed, one could argue that the Knights Who Say “Ni,” despite the brash ridiculousness of this plotline, encourage viewers to consider the linguistic confusion coincident with the Great Vowel Shift between 1350 and 1500: when King Arthur’s companion Sir Bedivere cannot pronounce “Ni” correctly, repeatedly stumbling over the syllable and saying “Nu” instead, such levity carries a very learned joke about the difficulties in pronouncing Middle English, both for medieval and modern speakers. At the very least, Monty Python and the Holy Grail, within the context of debates of medievalism versus neomedievalism, exhibits traits of both schools.
_{Pugh, T., Weisl, A. J. (2013). Medievalisms. London: Routledge, https://doi.org/10.4324/9780203108284}

However, the Wikipedia page on the Great Vowel Shift has this image:

As you can see, /eː/ and /ɛː/ and /iː/ on the left are fairly far away from /oː/ and /uː/ on the right, which I assume is significant, as merges are shown between vowel sounds that are next to each other.

Is there a genuine explanation of why Bedevere starts by saying "noo" rather than "ni", or does the joke not go that far? Is the parallel between the Great Vowel Shift and the "ni"/"nu" confusion simply that "vowels changed, so Bedevere struggled", or is there a reason for these two sounds to be confused?