They can use "the pool", i.e. the collection of NTP servers. They could (register a "vendor zone") and use "[0-3].systemd.pool.ntp.org". They can, technically,

They can use "the pool", i.e. the collection of NTP servers. They could (register a "vendor zone") and use "[0-3].systemd.pool.ntp.org". They can, technically, even use "[0-3].pool.ntp.org", but the NTP Pool Project is saying they cannot use [the specific hostname] "pool.ntp.org". That last part is what people are apparently not understanding. Also, OpenBSD blatantly ignores it and uses "pool.ntp.org" anyways.