Author here.
It's decentralized because the FromBank and the ToBank communicates directly over the Internet using TCP/IP+HTTPS+OpenPGP, which is different from a centralized communication system like SWIFT, where the FromBank would first send the message to ToBank's SWIFT address (so called "BIC"), and wait for ToBank to check their SWIFT "inbox".
Compare:
FromBank <-> ToBank (BankAPI, decentralized)
vs.
FromBank <-> SWIFT <-> ToBank (SWIFT, centralized)
(FromBank = Bank sending the message)
(ToBank = Bank receiving the message)
What happens when the beneficiary bank can't be contacted? Their servers may be down, they may be getting DDoSed, a production upgrade may have gone wrong and they're inadvertently dropping all messages.
That lumps a whole lot of complexity on FromBank, if the onus is on them to ensure ToBank correctly received the payload.
SWIFT takes care of all of this. You also have to guarantee to be connected to the SWIFT network for at least 7 hours per day to process your inbox, and new banks get silo'd in a test area for two months where they have to prove their systems are functioning properly within the SWIFT env before they're connected for real.
As long as FromBank gets its confirmation when it posts to SWIFT, job done. SWIFT takes care of ensuring it gets to ToBank.
We are already using SWIFT for much of our messages from/to banks, but it comes with a high cost and a lot more complexity than BankAPI, as messages are not sent directly from A to B, but from A to SWIFT, and then B must check their SWIFT "inbox" to receive the message from A.
With BankAPI, the response from the request comes directly from the beneficiary bank, meaning you know in real-time the message has been delivered. Compared with SWIFT, you only know SWIFT has received the message in real-time.
If the beneficiary bank can't be contacted for what ever reason, the sending bank (FromBank) simply keeps on trying until the beneficiary bank (ToBank) are back online again.
The problem with DDoS is a valid concern, but given the banks Internet banking services are also accessed by the banks users via the Internet, they are already dependent on the Internet.
If you don't need real-time messaging and if cost nor complexity is a concern, then you probably won't find BankAPI interesting.
