The Rashba contrasts an individual who tells the Goyim he believes in AZ and the Yidden that he is Frum, where we can assume that his true beliefs are not AZ, with a habitual Mechalel Shabbos.
The source of the commonly accepted issur on yayin of a m”S is that Rashb”a (quoted in B”Y) towards the end (after the part you refer to), who writes, “??? ????? ???? ????? ??????? ?? ????? ????? ???? ??????? ?”? ????? ??? ??? ????? ??? ???”