You are not the first person Ive heard this “distinction” between a prenup and kesuba from.
Maybe because the distinction is true?
Reminds me of waiting in line for the keilim mikvah on erev Yom Tov. A woman was waiting to toivel some plastic containers, and some guy was making fun of her. I pointed out that that some poskim do require it, because the same reason the Gemara uses to require tevilah for glass also applies to plastic.
So, the guy goes on an anti-chumrah rant, and yells, “People say not to eat strawberries, but Hashem created them!”.
I told him that Hashem also created the sheratzim in the strawberries, and He also created chazir, but we can’t eat those either.
He said, “You’re the second person to give me that answer!”.