@flyer
I just checked a fourth Chumash (המאור edition), and this one does have the piece you are referring to. (Seems like they added it in.) It is a quote from the Divrei Dovid, the Taz’s פירוש on חומש רש”י, who in turn is quoting something he saw as a boy in an old manuscript. And immediately afterwards, in square brackets, the copyist makes the same point I made: that this is definitely a מדרש תנחומא quoted by the ילקוט שמעוני.
Interestingly, the אוצר הראשונים חומש brings this pshat (that Rebbi Yitzchok was Rashi’s father) from ר’ יהודה החסיד, author of Sefer Chassidim. (This was probably what the manuscript the Taz saw was based on.) Strangely, he says וזאת מימרא דר’ יצחק כל בעלי מדרשים בדקו אחריה ולא מצאו מקומה (!) אך קבלה היא (!) שרבינו שלמה אמרה בשם אביו.