Was de Leon a Student of Paras?

In page 240, Comments & Cases on Partnership, Agency, & Trusts, 2010 Ed., the authors gave the following example to explain Article 1834:

Why the switch from D to X? My take is that the authors, while in law school, must have cut their legal teeth on the textbook of Paras.

Here's the explanation of Paras for the same article, from page 717 of Civil Code of the Philippines Annotated, Vol. V, 2008 Ed.