On my last trip to Thailand, I went across the Kingdom from West to East. On the way, I visited many ancient stone buildings  (ปราสาทหิน) enough to say that (Hindi and Buddhist) cultures moved from West to East. And it is extreme unlikely that the ancient Kmer expanded westward. In another word, ancient Kmer was not a civilization originator of the region.

I saw in many museums signs saying something like "in the ancient Kmer style" (แบบเขมรโบราณ). This is possibly a gross error and need urgent correction so we don't spread the error (among those who visited the museums). Better history is best served with "in the Khom style" (แบบขอม) and let more studies and evidence on Khom clarify who they were.