The site compares it to Rust and Go but to me the comparison is AssemblyScript. It’s also WASM-native and new with relatively little ecosystem around it. But compared to Moonbit it’s a familiar language to anyone that’s used TypeScript. So why use Moonbit over AssemblyScript?
Because Moonbit is a modern language, while AssemblyScript is carrying forward the mistakes of the past. For example, Moonbit supports pattern matching and most language constructs are expressions. AS doesn't have pattern matching and consists primarily of statements. Moonbit has algebraic data types; it's not clear to me that AS does.
There might be other differences at runtime, but it's difficult to tell from just the website.