I thought the same thing as I was reading it, but I think they are probably using larger block sizes than the SSD's blocks for better compression. I'm not certain though.
Edit: reading other comments, it looks like their block size is only 64KiB, so this isn't the case, so I don't know the answer. I can only think that perhaps it is an issue because ZFS doesn't deallocate the freed blocks quickly enough and they are making changes fast, meaning a significant amount of disk space is used by blocks that are about to be TRIMed but haven't been yet.
Edit: reading other comments, it looks like their block size is only 64KiB, so this isn't the case, so I don't know the answer. I can only think that perhaps it is an issue because ZFS doesn't deallocate the freed blocks quickly enough and they are making changes fast, meaning a significant amount of disk space is used by blocks that are about to be TRIMed but haven't been yet.