This page on Nedarim 20 is discussing the stringency of a husband avoiding thinking of other women during sex. https://dafyomi.co.il/nedarim/halachah/nd-hl-020.htm
When it comes to the Poskim discussing rebellious children being born from improper sex, it refers to Jacob, Leah (the deceptive wedding night) and the birth of Reuben, and quotes Magen Avraham: "Heaven forbid that one of the Shevatim was a rebel! We must say that the problem is only when he saw Plonis on a bed, and later thought that he was with Plonis. Here, Yakov saw Leah and intended to have Bi'ah with her, just he erred about her name."
From the context, am I right to understand that the mention of 'Plonis' is simply a random woman's name that they are using hypothetically for anyone in this situation?