I don't agree with this argument. Any of it.
The S is not only clocked higher, it has higher on-chip bandwidth (ESRAM). Its HDD also performs slightly better.
While it's true that in most games the difference is only 1-2fps difference, a few titles have shown up to a 5fps difference in performance in certain sections. If they don't test the base unit, how can we know if it's one of those titles that shows a larger performance gap.
Furthermore, the Base Xbox One was on the market for 3 years before the S was released. I don't know what Microsoft's sales numbers are, but I would bet there are a lot more owners of Base Model Xbox Ones than there are of Xbox One Xs. If that's your largest customer group, shouldn't you make sure that games released for their consoles are playable?