I assumed he might have meant that faking something like this would be the act of someone who hates Geno and likes to see Geno fans squirm, which I agree.
In defense of that argument, Geno is the most hated character in speculation right now.
It's possible that they might make Doomguy's fans squirm as well. I feel like if they were Geno haters, they'd probably only make it involving a fake playable Geno, or would only make it a photoshopped Geno costume to put some of Geno's fans down and make it believe that Geno has been disconfirmed as a playable character. After seeing the video, they would be indirectly doing it because all we see is a Cacodemon and Mallow Mii costume. No Doomguy or Geno in sight, nor do we know if Doomguy or Geno are Mii costumes. I'm hoping the costumes ARE real, and I'm hoping that Doomguy and Geno are playable characters (being part of the DLC).
If Doomguy is the 5th and Geno is also confirmed as a DLC character, I don't think Geno would be part of the fighters pass (as in being a bonus character for the fighters pass), but would possibly be the next starter in the new DLC lineup; what I mean by this is Doomguy is the 5th character, also the ending of the fighters pass. For me, that means Geno won't be instant with the fighters pass, I believe he'll be on his own (so far until new DLC comes) like Piranha Plant is with being solo/not being a part of the fighters pass. It's possible there's a 2nd fighters pass, and if there is, I see Geno being the first of the 2nd fighters pass.
Other than that, if this is fake, props to the person that made it because that Mallow Mii costume looks legit and nicely made.