The Sun Warrior Ruins, architecturally and geographically are based on Mesoamerica, not Angkor Wat and the Khmer.
The temple we see on the right hand side when Appa flies in, is based on a Mayan pyramid/temple. The location, on the other hand, with the high mountains surrounding a lush jungle, are more reminiscent of the Incas i.e. compare Machu Picchu.
The Sun Warriors were descendents of the most ancient Firebending culture. The designers looked at Mayan ruins for inspiration. (Art Book, p.156)