As local NVMe storage does not have any interaction with the "classic" block device mapping APIs (the storage shows up as a PCI device, the same way that a GPU or FPGA does, and it doesn't matter in any way how the block device mapping is set up), there is no reason to use "ephemeral" to describe it.

Said more directly: no, it is not ephemeral. It is local storage that is tied to the life cycle of the instance.