As long as you follow the 'protocol' you can be sure the orders matched the second the taker's order is confirmed by a single delegate .. (though not with 100% confidence)
You have 100% confidence after 51 delegates the block ..
I read the link but it didn't cover this.
Thinking out loud here...
My main point is that the orderbook is state which the validity of matched orders depends on directly, but the state of the orderbook depends on the history of the blockchain (as a sequence of replayed operations). Therefore, to match any order with 100% certainty, each delegate would need to replay the entire blockchain in order to rebuild the current orderbook...
...I guess that's actually what they're doing in a continuous and ongoing fashion? They all have a current state, which is the head block plus the mempool, therefore they have the orderbooks. So, to confirm any given order is matched correctly, every delegate must in fact match that order itself to see if it agrees with it's own state? But, since each delegate works on a new block, that means they have to check back and do 51 blocks worth of order matching?
Or am I missing something here?