Both links are the same, SWC in this context is probably Speedy Web Compiler. It transpiles really fast but doesn't do any type checks.
What's the point of using it during development, then?
Typechecking is not: the browser doesn't care about it, it's mainly to help the developer verify its code.
So to speed-up the build during development (to have faster iterations) the idea is often to make the building process only about the build by removing "unnecessary" steps like type-checking from it, while having a separate linting / typechecking etc. process, which could even run in parallel - but not be necessary to be able to test the application.
This is often done by using tools like a bundler (e.g. esbuild) or a transpiler (babel, swc) to erase the types without checking them in your bundling process.