The context they mean is probably the adjacent "u", which is true and is involved in the strict rules you speak of.

There is ambiguity with "x", though. I don't know if there are strict rules surrounding it. I know of 3 different sounds for it:

* Like Spanish "j", English "h" as in México, Xavier, Oaxaca

* Like Spanish "ch", English "sh" as in Xoloitzcuintle, Xela

* Like English "x" as in excepción, exacto