Yes, that's correct. ALL of the metadata from that point forward gets placed on the cache.

That is, until you fill it up - which you could.

At that point new metadata goes onto the spinning disk vdevs, as it did prior to the cache. As files age in and out, space gets freed on the cache and some new metadata makes it back on there.

So if you fill it up, it behaves more like a cache.

ALSO, you can add multiple metadata caches to a pool ... so if you fill one up, you can add another ...