I’m Cambodian, I can tell all Thai people that back during Khmer Empire, the empire wasn’t just exist with Khmer ethnic alone and it’s a common sense to think that way since the word Khom itself to Khmer people refer to ខម (ខ្មែរ-មន) Khmer and Mon Kingdom which merge to become a strong state as they have the same sense respect for Hinduism and people are closely connected in trade. However, khmer enpire also consist of Laos, Thai and Cham people which are controlled and under Khmer dominance since they don’t have an established state except for Cham people losing to Khmer because of war. During that time, Khmer, Mon, Cham, Lao, and Thai people living in peace until Javaraman VII passed away, that is when the Khmer empire and other power dominions got separated into their own kingdom, likewise even Laos people know that it was Khmer king help establish Luang Prabang city for Laos after Laos divided from Khmer. Not to mention all Cham, Java, Daiviet, China and India are all have documented that Khmer is the indigenous group just like the inscription in Angkor Borei which shows that about Khmer history.