The following is not profound but it is the simplest answer to a simple question, that does not seem to have been clearly explicated:
Abraham could serve them whatever he wanted (except for a limb torn off of a live animal) since the Torah which is interpreted as prohibiting meat cooked in milk had not yet been given.
Although one will find stray comments attributing observance of post-Sinaic laws to various pre-Sinaic characters, these do not seem to have been the intent of the OP, so addressing them is really irrelevant.
That being said, essentially all post-Talmudic commentators who focus on the simple textually supportable understanding of Scripture, emphasize that these aforementioned statements about pre-Sinaic behavior are homiletic in nature. Some clarify that the intent is to their general upstanding ethical status.