It belongs in the filesystem in exactly the same way that volume management belongs in the filesystem.
ZFS includes volume management, _naturally_. I say naturally, but that was a radical idea 15 years ago. Even now it's not universally accepted, but it's quite correct!
I'll add that ZFS is also quite concerned with write patterns IIRC, and thus wear leveling.
The author of the article seems to assume we should all be trusting SSD or "hybrid storage" firmware to properly handle this sort of thing for us like nice black boxes.
I think this was one of the major problems ZFS was designed to solve. To make storage hardware more simple the idea was to move a lot of this logic into the OS (especially as large RAM sizes got cheaper). It's why having a RAID controller sitting under your vdevs is advised against.
ZFS includes volume management, _naturally_. I say naturally, but that was a radical idea 15 years ago. Even now it's not universally accepted, but it's quite correct!