Are Angels non-kosher?

Genesis 18 came round in the cycle of readings the other day (readers with a Missal will thus know how far my backlog goes).  The Lord appears to Abraham by the oaks of Mamre. At some times the number of visitors appears to be three (taken to be angels), at other times it appears to be one (the one Lord). Abraham hastens to make food ready.

Then he took curds, and milk, and the calf which he had prepared, and set it before them; and he stood by them under the tree while they ate.

The consumption of dairy with meat or after meat is forbidden under Jewish law, based on texts which forbid the consumption of kids boiled in their mother's milk (Exodus 34:26, Exodus 23:19, Deuteronomy 14:21).  The Torah is said to be eternal, so why does Abraham serve this forbidden food?  That question led me to this answer from a Jewish advice website.