JSX is, imho, more akin to a macro; the syntax is replaced with simple expressions. Technically, the JSX syntax isn't even necessary to use React, though most people prefer it.
Svelte bakes itself fully into your code in a way that you can't do without the compiler and still be using svelte.
The point is that Svelte is a full compiler- you cannot "use svelte" without it.
JSX is an essential, yet entirely optional, concept in React. The output of the transform is incidental to the actual behavior of React, the library.
As such, I wouldn't think it warrants the term "JSXscript" or whatever the post I replied to used.
Depending on how your tool chain is set up, you don't need a custom file extension like ".jsx" for files that need the transform, and React doesn't care about file extensions at all.
Svelte, on the other hand, pushes you to use the ".svelte" extension, mixing and matching tags, changing scoping rules, and so forth. If you were to write by hand what it puts out, you're not really using svelte anymore. It is, in this sense, elevated itself to a language superset.
I'm not arguing one is better or worse, just pointing out why I think they are sufficiently different to quibble over semantics.