Of course HPE doesn't write their own firmware from scratch. It's likely just whitelabeled by the drive manufacturer. HPE is just a reseller of existing OEM drives, like all other enterprise server manufacturers are.
There might have been some small changes, but the bulk of the code is likely copy pasted from a common codebase that is shared across models or even families. There's no way they're rewriting the entire controller code just for one customer, it doesn't make sense technically or from a business perspective.
