The original source for this is the Meshech Chochmah which is quoted and expanded upon by R Moshe Sternbuch in MoAdim UzManim Vol 4 Chapter 319 page 139 original

... thus Avraham Avinu who fed the visitors meat cooked with milk, means it was BP which is not restricted by the meat and milk prohibition

accordingly the angels were rebuffed since they themselves had authorised Avraham's identity as a Jew [by eating the meat and milk combination he cooked for them] and the Torah is legitimately the domain of the Yidden

רק השחיטה מתיר בבן פקועה ושרי בחלב - since Avraham's Shechitah permitted the BP permitting it to be cooked with milk