Hi, I wanted to give you an update that I'll be taking over as Wiki Manager for this wiki from Shrev64. Please feel free to send me a message if you need any technical help or want to reach out to Fandom!
This is some background royalty-free dribble that YouTube's content ID picked up and is used in a total of 4 videos, not "every Markiplier video", two of which are linked to the game itself, not Mark's choice of background noise, therefor, it doesn't meet any of our requirements to even be categorized on the wiki, especially not as a stand alone page.
Wikis are for listing facts, not conspiracy theories. If you have any proof, you can list it under this message thread. You should have done so in the first place on your article as references.
I've actually looked into this and found absolutely nothing, leading me to believe you've either been mislead or are just trying to start rumors for nor reason.
There are no discussions anywhere about Markiplier's Deltarune on most of his common socials, most people just mention that they wondered if Mark would ever play Deltarune on stream. There's no reason for him to hide ever posting this video, yet neither he nor anyone else remembers nor ever mentions it. You've said it yourself that the thumbnail was doctored, but the supposed screenshot of the actual clip isn't found anywhere. Another thing that makes me believe it's fake is the quality of it; if this supposedly took place in 2018, why does Mark's camera quality look so low-rez, even compared with the rest of the video, along with video's missing header.
Unless you can provide undeniable evidence of this ever existing, we will not be listing it on the wiki. Everything on here has some kind of source or reference.
You're required to fill out the page with basic info, properly categorize it and add an infobox for it to be left as a substantial article, a few lines don't cover basic requirements. For a comparison, look up other game-related articles.