I've only watched part of GradeA's video thus far (up to where he said Santoro plagiarizes), but I feel like he went too far with Markiplier.
Now, some of his criticisms are legitimate, like how rambly and vague Mark's video was, or the more subscribers means more understanding argument. Others, though, show his lack of understanding about Markiplier.
For example – Markimoo. That's basically his community-assigned nick name. It's not something he pulled out of his ass. His fan base loves to call him that.
The other thing I had a major objection to is how he basically insisted he was an idiot, and called him a retard multiple times over. He was, before he got popular on YouTube, going to become an engineer. I'm pretty sure you have to be above retard levels of intelligence to have a shot at that, and by how Markiplier acts when it comes up he really does. This was shown during one let's play he did of a game – don't remember the name, it was about bridge construction – and how he flew through it with relative ease, and remembered a lot about engineering. He nerded out for a good portion of that series, because he was in his element.
On the more subjective side of things, I felt Mark's video was more of a vent kinda of thing than anything else. That's just me though, and I might be wrong there, but it'd explain the disorganization and lack of scripting. He occasionally makes videos like that when he has something on his mind.