* NTP Pool welcomes open source projects to use their servers
* They want vendors to register a vendor prefix
so there are two options, either systemd is a vendor, and can register and use systemd.pool.ntp.org, or it isn't, and can use pool.ntp.org
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.