@flyer
I just checked a fourth Chumash (????? 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 ????? on ???? ??”?, 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 ???? ?????? quoted by the ????? ??????.
Interestingly, the ???? ???????? ???? brings this pshat (that Rebbi Yitzchok was Rashi’s father) from ?’ ????? ?????, author of Sefer Chassidim. (This was probably what the manuscript the Taz saw was based on.) Strangely, he says ???? ????? ??’ ???? ?? ???? ?????? ???? ????? ??? ???? ????? (!) ?? ???? ??? (!) ?????? ???? ???? ??? ????.