Yeah I don't know how they arrive at that conclusion honestly.
When playing a nice game and you've locked the dude in for a "next turn he's dead no matter what" play, Zilliax always pops onto the board healing them for 3, taunting up, and destroying something of yours. The inverse also happens when you're dreaming of a topdeck Zilliax because its the only thing that will save you and it happens. The inability to interact with it just feels like Ice Block tbh.
Half the decks being brought to HCT this weekend sport the neutral legendary.
A stat that is backed up globally with data from HSReplay which has Zilliax sitting at being used in 48.7% of all decks.
Also just the fact that they said it isn't "awful" to play against is like saying it's better to lose one leg than both. While not wrong, it's a dumb statement to make. They do also state that they like it because it fits into tons of different decks for different reasons each with a different way to play it, but that's also what just makes it an annoying card.
IMO remove Magnetic or bump the cost up. Could help some of the nonsense and potentially remove it from being played in some decks. Some of those Hunter lists feel like they wouldn't be as good if he didn't have Magnetic, though don't quote me on that because its just a feeling and not something I have any stats on hand to back up.