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Why was 33...Rxa2 an inaccuracy?

Because d4 then Bd5+ is near immediate mate.
That's interesting. I wonder what makes the labeling for that inaccurate when move 22...Rf5 looks a lot more like a blunder. Bg3+ could have been played.
@PiecesMovedBackwards said in #1:
> I suck at chess, so can someone explain this for me?

It's not an inaccuracy in terms of it's winnijg or not. It's is still winning

It's an accuracy if your goal is to meet engine expectation with the most efficient way to beat the opponent.

Most moves, if not any, at this position is almost sure win. I said almost because there's still a potential draw If you are careless.

In my book, at this point, who cares if it's an inaccuracy. Most GMs have engine inaccuracies too.
@noobie519 said in #4:
> It's not an inaccuracy in terms of it's winnijg or not. It's is still winning
>
> It's an accuracy if your goal is to meet engine expectation with the most efficient way to beat the opponent.
>
> Most moves, if not any, at this position is almost sure win. I said almost because there's still a potential draw If you are careless.
>
> In my book, at this point, who cares if it's an inaccuracy. Most GMs have engine inaccuracies too.

The question had more to do with "selective mistake labeling". You can have a family of inaccuracies (small mistake), a family of mistakes, and a family of big mistakes (blunders). I would think the Bg3+ move would be more crucial to point out than what you are confirming doesn't matter.
By capturing a2, White's king is pinned, and their only available move is the rook. Meaning, they can slam that rook down to f8, and if you capture it it's stalemete. And if you don't capture it, they just chase the king around and draw by perpetual check. White still being able to move a2 is vital for black to win.
It's not a draw, the engine still sees a mate for black after Rf8+, the point being that 35. Rf8+ Kh7 36. Rh8+ Kg6 37. Rh6+ Bxh6! breaks out of the stalemate trap by allowing the king to go to g1.

But yes, it was dangerous to take on a2.