We do not know for sure that daylily leaves (or that all daylily leaves all the time) are toxic to cats. There are many (including me) who have cats that eat daylily leaves and they suffer no effects.
A possible problem with the online statements is that many gardeners, etc., call daylilies (Hemerocallis) - lilies and some gardeners call lilies (Lilium) - daylilies.
Lilium apparently are toxic.
