You are right though, a checksum used this way provides no protection over bit errors or misdirected write/reads. But, if you assume no bit errors, a checksum can provide power-loss protection as long as there's a fallback.
But doesn't this just move the problem somewhere else? Yes. In this case it moves error detection onto the block device. But it turns out performing error detection/correction at the block device level is simpler and more effective. Most NAND flash components even have built-in ECC hardware for this specific purpose.