I think that’s always the case? I mean, the focal length is actually an internal physical measurement in mm, as I understand it, not some sort of oblique reference to field of view, even though that’s how it tends to be used.
I think Sigma are just trying say that it’s not giving a 17mm-on-full-frame field of view on your aps-c camera… with the emphasis on ‘trying’. 
That len would actually be ~12mm-ish.
No one in the interchangeable lens camera market labels APS-C or M4/3 lens with their ‘35mm equivalent’ focal length. (umm… except… pentax Q… possibly?). That’s a compact camera trick - I suppose they think (with some justification) that it’s too hard for buyers to know what field of view they’re actually buying.