Curious, what is the evidence that the Samekh was like an "X"?
The first evidences I noticed are that the Samekh is in the same place in the alphabet in both Hebrew and Greek where the Xi has the “X” sound. Secondly, both in archaic Hebrew and archaic Greek, the letter has the same shape. Only later I noticed that the name Artaxerxes uses a Samekh for the second “x” in Hebrew.
Admittedly, these are not proof, but suggestive.
The Greeks got their alphabet from the Phoenicians, and admitted to that. The Phoenicians from the Hebrews, so I’m assuming that in the borrowing that the phonemes stayed the same, at least at first.