Yup, it is SFTP for both sending files (initiating payments) and receiving files (getting balances, confirmations, etc). This is how most programmatic money movement was happening under the hood at SVB (and many other banks for that matter).
I'm not surprised, I always wondered how that happened on the backed nowadays. I figured they had upgraded over the years and made it a little more streamlined. no wonder it still takes some time to move stuff around... I wonder how is the SFTP discovery / account process happen?