He didn't say anything factually wrong like math calculations, he expressed his opinion and even if silly, it was harmless.
Okay
He got facts about the lores of both franchises wrong
One major reason the video gets mocked is because how you can use facts to debunk almost everything the guy brings up as evidence
The other major thing being that he jumps to conclusions with no real reasoning
He claims to be the smartest channel on gaming
He never encourages people to try and debate the theory, just think from a different perspective so that you can ignore the things that debunk it
He does this all the time but just so happened to do it on something that was a major hit on the internet so everything would be fresh in their minds
If he gets things wrong and people call him out, he does not get to cry and whine about negativity
EDIT: Oh and there's the fact his entire theory falls flat because Matpat is a ****ing moron who doesn't know bones are organic