We are planning to make some important changes in core, so the HTML cache improvement waits yet. Nothing is changed yet. It's only caches the forum home page. It's used if you have hundreds of forums, then the forum home is too large and may affect the performance. In regular forums with dozens of forums the Object, Memory and Users caches do  a great job, so no need to enable HTML cache or other cache plugins.

Thanks Robert for the reply.

So if i get this right, no topics are cached as html static pages, only the forums home (the page we put the shortcode in) ?

Yes, you got it right.