Why does the gemara on daf hey amud beis consider it a given that chilul shabbos is an issur kares? Isn't it an issur skillah?
Bear in mind that every Chiyuv Sekilah is Chayav Kares too. So Shabbos incorporates both Sekilah and Kares.
Why does the Sugya pick on Kares (and not Sekilah), you will probably ask? The answer is because our Sugya is concerned with the Isur of Achos Ishto (and most of the other Arayos, which are Chayav Kares), and which we think the Mitzvah of Yibum would overrule, were it not for "Alehah".
So we cite the precedent of Kibud Av va'Eim, which, if not for the Gezeiras ha'Kasuv, would override Shabbos, which, like Achos Ishah, is Chayav Kares, too.
Be'Virchas Kol Tuv.