Why does it mark it wrong if you use the plural verb form with ni l'un ni l'autre?

Question

It's more obvious when we're talking about a positive sentence like:

l'un et l'autre viendront => both are coming => plural.
l'un ou l'autre viendra => only one is coming => singular.

But when it is negative:

ni l'un ni l'autre = pas l'un et pas l'autre => so it can be plural no?

Multiple sources suggest both singular and plural verb forms are possible for ni l'un ni l'autre:

http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=1490#:~:text=Lorsque%20le%20sujet%20est%20ni,deux%20noms%20coordonn%C3%A9s%2C%20est%20pluriel.

https://www.gymglish.com/fr/frantastique-orthographe/regles-orthographe/ni-lun-ni-lautre-nanont-raison

https://www.btb.termiumplus.gc.ca/tpv2guides/guides/clefsfp/index-fra.html?lang=fra&lettr=indx_catlog_n&page=9WmCJCDuX_J0.html

Accepted Answer

Bonjour Niall !
Indeed, your feedback brought our attention to this very interesting point that wasn't mentioned in our lesson. Thanks to you, it's now been remedied :)
In the specific case you mentioned, both plural and singular could indeed be acceptable (see "new" lesson!), but we decided to change this question to remove any ambiguity, especially as we're not testing conjugation here :) This question has therefore been replaced.
Thank you very much for your feedback and I hope that's helpful!Bonne journée !