This is rasbperry pi 1 the post is about. It's not a super computer. Running updates and unpacking archives chokes the SD and CPU on official raspbian with ext4. It only has 512MB of memory, so ZFS would be killing it.
Yeah, the pi 1 is really slow for a lot of things these days. While the post mentioned they started this on a pi 1B+, everything else is about running things on model 3 units.
Yes, regardless whether RPI1 or 3. These boards are quite weak. I wouldn't want file system to one of the biggest bottlenecks on a running system. I had BRTFS on rpi3 before and don't recommend it. Both FS are CoW.
It sounds like FreeBSD also works on the Pi 3 and Pi 4: