While luck may have played a part on this one as well, I think for the most part, it's because they did a good job -- but please don't let this create a false sense of security on you though haha. After all, it's a custodial platform at the end of the day.
A partial payments exploit works by assuming a company has an improperly configured system that only reads the amount field of an XRP transaction, which is set to a high amount.
In reality, the exploiter sends over a much smaller amount specified in another transaction field and aims to receive credit for the difference.
Ardoino, however, noted the attack failed as “Bitfinex properly handles the ‘delivered_amount’ data field.
Other exchanges should take note of this exploit as well because apparently, they did this with binance too and thankfully failed hence, it wouldn't be odd to think the perps will try this again on other exchanges.
According to blockchain data, the attacker also tried an attack on Binance with a 58.9 billion XRP transfer, which similarly failed.
On another note, next time I think it would be better to create another thread for this