According to all the dev docs out there, there isn’t a way to disable one and not the other so the bank will have to put touchID on hold for all of its other users while they ‘warm up’ to the security of FaceID.
Stop spreading misinformation. It is simple to not support one or the other, you just don't request biometric authentication if you see FaceId is supported, or if you only want to support FaceId you don't request authentication if TouchId is supported. Once you call the API then yes either will authenticate. You just don't call the API if you see FaceID is the biometric type supported.