After poring over Mother videos for 5+ hours I've concluded that Dessknight is real
Plenty have said that it feels derivative of the Masked Man plot in Mother 3, but I think Toby doesn't care because there's a very specific kind of experience Toby is seeking to create.
in Mother 3 there's this dummied out boss known colloquially as "Lucas's Nightmare", as well as a number of other dummied out scenes that still exist in the game data, and it's a matter of public record that the head of the project wanted Mother 3, especially its climax, to be darker than the previous two games, but at the same time the ratings classifications for games was getting stricter
and it gets spelled out for us in Chapter 3 that the game Tenna is giving us had all the dark stuff dummied out
So I think the desired experience Toby wants to make with the Weird Route is to simulate what it'd be like if you could exploit glitches and bugs and sequence breaks (such as, if you keep spamming this one spell a character can't cast yet, the game bugs out and she casts it anyway) to access all the dummied out scenes of a game in a way that doesn't render it unwinnable.
Dess is the Knight because that's the final boss fight of the game, and while Weird Route doesn't change the ending, it will let you experience a much more fucked up version of that boss fight that'll spell out a lot of stuff that prior to that only existed as speculations and interpretations, comparable to the "the final fight of mother is performing an abortion on Gigyas before its born" theory