How can I get comments from swc_ecma_parser

The comments are indeed not part of the AST, but the parser handles them separately. The third argument of Parser::new is comments: Option<&'a dyn Comments>. In your code you provide Default::default() here, which is simply None for an Option, suppressing any comment parsing. You can instead create one of the implementations of the Comments trait, such as SingleThreadedComments:

let comments: swc_common::comments::SingleThreadedComments = Default::default();
let mut parser = Parser::new(
    Syntax::Typescript(ts_config),
    StringInput::from(&*fm),
    Some(&comments),
);

After invoking the parser, comments should be updated (the API uses a shared reference for reasons). You should be able to mutably iterate through the comments using borrow_all_mut.

Finally, it seems like swc::Compiler::print is the method you will want to use to convert the AST back into code. Again, this method accepts comments: Option<&dyn Comments> as its last argument.