Clarify that all recipient keys must be on the same curve

[jakubkoci]

The section 5.1.9. ECDH-1PU key wrapping and common protected headers says:

epk: generated once for all recipient keys. It MUST be of the same type and curve as all recipient keys since kdf with the sender key must be on the same curve.

Does it implicitly mean that all recipient keys MUST be the same type? In other words, we should check if all recipient keys are on the same curve before we proceed, and throw an error if they're not. Do I understand that correctly?