JSX is syntactic sugar for JS[0]. C++ and ASM (actually, _which_ ASM?) are different languages.
A less flippant analogy might be macro support in Rust. It's clearly part of Rust, but the syntax is completely different and it requires IDEs to have completely separate processing just to handle it. I wouldn't consider that to be mere syntactic sugar either.
[1] Yes, yes, wasm
JS and TS aren't isomorphic (there's no inverse morphism once you go TS->JS that can bring the resulting JS back to the original TS) hence why TS is a different language (even if a superset of and compiled to JS).
They are not
Or, if they are, they are in the same way of a custom XML templating language that can be translated to a specific javascript library implementation and then back to XML
In the same way, is Mustache isomorphic?
"{{title}} spends {{calc}}"
can be translated to `${title} spends ${calc}`
and back to "{{title}} spends {{calc}}"