Both, but perhaps more often neither.

From what I've seen, sometimes the computer correctly assesses that the "bad" move opens up some kind of "checkmate in 45 moves" that could technically happen, but requires the opponent to see it 45 moves ahead of time and play something that would otherwise appear to be completely sub-optimal until something like 35 moves in, at which point normal peak grandmasters would finally go "oh okay now I get the point of all of that confusing behavior, and I can now see that I'm going to get mated in 10 moves".

So, the computer is "right" - that move is worse if you're playing a supercomputer. But it's "wrong" because that same move is better as long as you're playing a human, who will never be able to see an absurd thread-the-needle forced play 45-75 moves ahead.

That said, this probably isn't what GP was referring to, as it wouldn't lead to an assignment of a "brilliant" move simply for failing to see the impossible-to-actually-play line.

It does happen that the engine doesn't immediately see that a line is best, but that's getting very rare those days. It was funny in certain positions a few years back to see the engine "change its mind" including in older games where some grandmaster found a line that was particularly brilliant, completely counter-intuitive even for an engine, AND correct.

But mostly what happens is that a move isn't so good, but it isn't so bad either, and as the computer will tell you it is sub-optimal, a human won't be able to refute it in finite time and his practical (as opposed to theoretical) chances are reduced. One great recent example of that is Pentala Harikrishna's recent queen sacrifice in the world cup, amazing conception of a move that the computer say is borderline incorrect, but leads to such complications and a very uncomfortable position for his opponent that it was practically a great choice.

It can be either one. In closed positions, it is often the latter.

It's only the later if it's a weak browser engine, and it's early enough in the game that the player had studied the position with a cloud engine.

Here’s a chess stackexchange of positions that stump engines

https://chess.stackexchange.com/questions/29716/positions-th...

It basically comes down to “ideas that are rare enough that they were never programmed into a chess engine”.

Blockades or positions where no progress is possible are a common theme. Engines will often keep tree searching where a human sees an obvious repeating pattern.

Here’s also an example where 2 engines are playing, and deep mind finds a move that I think would be obvious to most grandmasters, yet stockfish misses it https://youtu.be/lFXJWPhDsSY?si=zaLQR6sWdEJBMbIO

That being said, I’m not sure that this necessarily correlates with brilliancy. There are a few of these that I would probably get in classical time and I’m not a particularly brilliant player.

It used to happen way more often with Magnus and classical versions of Stockfish from pre Alpha Zero/Leela Zero days. Since NN Stockfish I don't think it happens anymore.

Maybe he means not the best move but an equally almost strong move?

Because ya, that doesn't happen lol.