Never mind. I asked in game. Turns out that I needed to attune to the Aetheryte as part of the unrelated story quest line for Hard Mode Primals.

Thing is, the Relic Reborn quest line makes no mention of the need for that.

Edit: Yes Jerec, that's exactly the issue, I posted the above before reading your reply. Thanks again.

Edit 2: I would still consider it a documentation bug/issue, that the Relic Reborn quest line makes no effort to remind people that the need to do the unrelated Aetheryte attunement quest (for people like me who'd forgotten about that).