El Oso said:
Two thoughts here:
1. You're putting the cart before the horse. Rabbinic law does say these foods were not permitted, but rabbinic law did not exist yet.
2. The text doesn't specifically mention how the food was eaten. A careful reading of the Hebrew text in Genesis 18:8 shows the sequence: "He then brought some curds and milk and [then] the calf..."
"So he took butter and milk and the calf which he had prepared, and set it before them" NKJV
"And he took butter, and milk, and the calf which he had dressed, and set it before them; and he stood by them under the tree, and they did eat." KJV
"He then brought some curds and milk and the calf that had been prepared, and set these before them. While they ate, he stood near them under a tree." NIV
"Then he took curds and milk and the calf that he had prepared, and set it before them. And he stood by them under the tree while they ate." ECV
"And he took butter and milk, and the calf which he had dressed; and he set them before them, and they did eat, and he stood by them under the tree." Brenton's English Septuagint
"And he took curds, and cholov (milk), and the calf which he had prepared, and set it before them; and he stood by them under haetz, and they did eat." Orthodox Jewish Bible
No other source seems to agree with your careful reading of the Hebrew text. So I'm going to disregard it as in error.
And Jewish dietary law did not exist. That's the point. No Jew is going to do that.