but would it be reasonable to expect svelte-select to handle that so that consumers of this lib don't have to do all that?

Rollup builds atop Browserify and Webpack's lineage to make it possible to easily consume those packages, while looking to the future of JS modules.

As you've probably already figured out it's the last output definition that should be by legacy apps. With the above output configuration we allow consuming the component both from a modern app with a Node.js-based environment and from legacy apps.