It's not kept secret the documentation is available more or less to the "public", so is the SDK. You can also register for training, I underwent SWIFT training and I never worked for a bank, I'm not sure how it was arranged but I doubt it that strict of a process. There are 100's if not 1000's online and offline providers for SWIFT training from training bank employees on how to use the software as end users through IT specialists which deploy the software to developers which build applications on SWIFT.

To get a swift.com account you need to have details, name, institution code (BIC code for banks) and some additional details.

SWIFT doesn't handle the authorization the institution you are registering on behalf does which means it's probably even more easier to phish/social engineer yourself in.

Gaining access to swift.com credentials also shouldn't be that hard, especially if have already compromised the bank's network or any of it's employees.

To get access to the SWIFT documentation and other materials you also don't have to compromise your initial target which makes it even easier.

Registration to SWIFT's "Mystandards" is open to the public completely which both gives you access to quite a bit of information as well as allows to you participate in discussion groups which may lead to a few more vectors for social engineering.