Ah, nice find. They seem to imply plugins can be written in JS, with an API that _may_ be similar to Webpack, but definitely not drop-in compatible:
> We're planning on making Turbopack very flexible and extensible, but we're not planning 1:1 compatibility with Webpack … most Webpack plugins won't work out of the box with Turbopack. However, we're working on porting several of the most popular Webpack plugins to Turbopack.
It sounds like plugins will be a thing but existing webpack plugins will need to be ported.