The payment processing was literal parsing of data files on the local disks. It was where modern fintech interfaced with archaic banking technology. Apparently they ran benchmarks and concluded NodeJS was the most suitable for their requirements.

Not necessarily, you could have a few large requests that each do a lot of database work at the backend, that would also be high IO. IO is just any low level network or disk activity that is typically done by the kernel, not the userspace (nodejs) itself.