Yup, even the T1i had expanded ISO, but the thing here is that they've lowered their ISO rating, not that they have an expanded range. Their new expanded number was considered part of the basic ISO range on the 5D3 (and the 7D2).

At first I thought it's just marketing. Full-frame users expect lower noise levels when pixel peeping, than crop users. When they give us a crop density full-frame sensor, they feel a need to say "the ISO is good for you full-frame users up to 6400", rather than 16000 (with expansion to 51200), all because you expect less noise, even though you'd get that noise advantage back by downsampling to standard full frame resolutions.

But at ISO 6400 (expanded ISO to 12800) it's a full two stops less than the 5D3 and 7D2's sensitivities. The expanded range is less than the previous camera's base ranges.

Something has changed. They're throwing away sensitivity to gain something else. The drop in sensitivity *has* to be about low noise, and better DR, because otherwise it's a worse camera than the 5D3 for most people.