This has nothing to do with American english.
The latin alphabet, where X is pronounced as /ks/ (thus close to "ehks") from the even older Etruscan it was based on, dates all the way back to at least 600 BCE. Meanwhile, the use of "cross" dates from 11th-century Old English at the earliest, specifically for religious context. "Ehks" is not an "American thing", it's "cross" that's a Middle Ages thing.
The post you replied to was also a just joke, but if you want to make it personal then I don't know what to tell you.