I think that the two are often mixed up. This picture from the AHS database is what I've always thought was 'Flore Pleno':
https://www.daylilies.org/Dayl...
My 'Kwanso' has a muddled centre with petaloid stamens, not as tidy a flower as 'Flore Pleno' and more like the ID flower:
The flowers, at least on 'Kwanso', can be somewhat variable so it can probably help to look at several on a clump rather than just one.