hydrate:true tells the runtime to look at the existing content in the DOM instead of just appending the new content to the target you've specified.
More info here: https://svelte.dev/docs#Creating_a_component
In svelte-routing's example the server.js file renders the HTML first, so hydrate:true is required. If you don't want to use the server.js (e.g. you don't want to to do SSR), you don't need hydrate:true!
Install the dependencies...
cd svelte-routing-no-hydrate
npm install...then start Rollup:
npm run dev