Your boss is wrong haha! Webpack and other bundlers are used to reduce the size of the final page. they minify and do other stuff to achieve this. Vite also does this when you use the npm run build command, but it uses rollup (another similar tool) which can result in smaller sizes than webpack.
When you are just running the npm run dev it skips that however, because its completely unnecessary for development. That’s why its so fast, and provides a better experience.
Svelte does seem pretty good yeah. Its what I recommend to beginners, despite being into React myself. And yeah svelte-kit is using Vite under the hood 
Good luck!