Commentaire sur La Genèse 19:37

מואב MOAB — This daughter who was immodest openly proclaimed that the son was born of her father (מֵאָב) but the younger named her child in a euphemistic fashion and was rewarded for this at the time of Moses, as it is said regarding the children of Ammon, (Deuteronomy 2:19) “Do not contend with them” — in any manner at all — whereas in reference to Moab it (Scripture) only forbade waging war against them but permitted them (the Israelites) to vex them (Genesis Rabbah 51:11).

עד היום, the time frame mentioned refers to the period when the Torah was written by Moses.

ותקרא שמו מואב...ותקרא שמו בן עמי. They called the names of their children thus to show the world that the father was not someone who was an unworthy, sinful person. [from the point of view of Jewish law as it pertains to gentiles, a father may sleep with his daughter. Ed.]