That recommendation was written long time ago and it definitely needs an update. Some major Linux distributions replaced ntp with ntpsec, so that should be mentioned too.
FWIW, in my monitoring of the pool servers the most used implementations (counting IP addresses) are currently:
- ntp - 56%
- chrony - 24%
- ntpsec - 9%
- openntpd - 2%
As for the amount of handled NTP traffic, the biggest share probably has Cloudflare with their anycast addresses included in every pool zone. They run chrony with cfnts as a proxy, so I think most of the pool’s time service is provided by chrony, despite the pool’s recommendation.