I don't think that's necessarily true. Depending on how warped they were by the energies of the Abyss, they could well be entirely different--or even weaker (or stronger)--entities.
I'm pretty sure demon lords are still primordials. The Abyss is still part of the Elemental Tempest, after all - you can tell because all demons in the MM, up to and including Orcus, have the Elemental type, and types correspond to planes of origin:
Natural: Material Plane.
Fey: Feywild.
Shadow: Shadowfell.
Elemental: Elemental Tempest.
Immortal: Astral Sea.
Aberrant: Far Realm.
As for "primordial" being a new role a la "controller" or "brute" - God, I hope not. Roles should be descriptive of what the thing actually does in combat. Controllers control the battlefield, brutes are big and tough and hit hard, soldiers hold the line, lurkers lurk in ambush, skirmishers dance around and make hit-and-run attacks, artillery lay down heavy damage at range. None of these roles is tied to a specific monster type.
If WotC has thunk up a new role, that's great, but it shouldn't be explicitly tied to primordials. It should be independent and descriptive.
I'm pretty sure this was just an error. But if it wasn't, it was absolutely a mistake.