What they are describing is that feeling you get when you suddenly realize you are losing even though you are even in pieces, because you suddenly see your opposition has superior positioning and board control
It's not a blunder, because there wasn't one particular move where the game slipped away.
At high levels, blunders are rare.
A move that worsens the position in a way that's not so immediately obvious might be called a 'mistake' or 'inaccuracy'
By these standards, it's quite possible for a game between humans to be won or lost without a blunder on either side.
Hence why I talked about the feeling you get when you realise you have mis-evaluated rather than trying to define it precisely.
From the OP:
> But when AlphaZero plays, and you watch Stockfish's score, the reason it wins is that it makes moves Stockfish thinks is poor, so it rates AlphaZero's moves poorly, and then all of a sudden it has an oh shit! moment when it realises that AlphaZero is actually ahead, and its score jumps. It's really a look inside the computer's head while it's being beaten by a better player.
Sure - you get that feeling from blunders too. But take a look at the Stockfish vs Alpha Zero games - Stockfish doesn't blunder. It it just outplayed: https://www.chess.com/news/view/updated-alphazero-crushes-st...
If you watch from here in game one you can see the "oh shit" moment, as Stockfish's evaluation drops from +1 to even to -1: https://youtu.be/Q5EPqM8gS7k?t=255
Not quite, as that would imply that each game has at most one blunder. It's quite possible for two players to blunder back and forth multiple times.
A mistake is like a blunder but you're still winning, but it could be a blunder in a worse position. An inaccuracy is a bad move that doesn't cost you.