An unhelpful non-canonical answer - but in The Dying Earth RPG I'm pretty sure IOUN stones require attunement, which is presumably based on what Vance wrote in his stories.DMG does not mention attunement but SRD says Attunement. Which is officially true?
DMG p176:
The DMG does require attunement.DMG does not mention attunement but SRD says Attunement. Which is officially true?
Not in mine - early printing?
The DMG does require attunement.
You're just missing it - it's mentioned once in the header, not for each individual stone.