Did the biblical prophet Moses promise his firstborn son to idolatry?

 https://www.jpost.com/judaism/article-747272

In an unexpected passage in the Midrash, the sages recount a deal struck between Moses and his future father-in-law, Jethro. Moses had already decided to serve God when he came to Jethro to ask for Zipporah’s hand in marriage. Before giving his blessing, Jethro asks Moses for one undertaking.“What is it?” inquired Moses. 

Jethro explains: “Your first child should be for idol worship, from here onwards,” – referring to subsequent children – “they will be for the sake of Heaven.” 

When Moses said to Jethro, "Give me your daughter Tzipporah as a wife", Jethro answered, "If you do what I ask of you, I will give her to you as a wife." Moses said: "What do you ask?" Jethro replied: "Your first son must serve idolatry. The following sons may serve God in heaven." Moses accepted. Jethro said: "Swear." And Moses swore. [Mechilta d'Rabbi Yishmael 18:3]