That's only for deduplication.
ZFS also uses RAM for read through cache aka ARC. However, I’m not sure how noticeable the effect from increased RAM would be - I assume it mostly benefit for read patterns with high data reuse, which is not that common.
I do like to deduplicate my BitTorrent downloads/seeding directory with my media directories so I can edit metadata to my heart's content while still seeding forever without having to incur 2x storage usage. I tune the `recordsize` to 1MiB so it has vastly fewer blocks to keep track of compared to the default 128K, at the cost of any modification wasting very slightly more space. Really not a big deal though when talking about multi-gibibyte media containers, multi-megapixel art embeds, etc.