• Module bundlers like Rollup can do tree-shaking to create a bundle that includes only the code that’s used.
• If a member is imported from a CommonJS module using the correct syntax, the tree shaking can include that module.

So the first part goes to the heart of ESM being statically analyzable, so that an all-ESM module graph can get pruned so that a bundle with only the used code is generated.

The second part I assume includes code like import { shuffle } from 'underscore' (as opposed to import _ from 'underscore') where we know that shuffle is the member being imported and so therefore the module graph can include just shuffle rather than all of underscore. See also #100 (comment)

Use case 12.