They only had a 10mbit link. Apparently 50mbit/s was the amount of traffic they received.
Mostly everyone could handle this, not just CloudFlare.
Re:AWS IPv4 Estate Now Worth $4.5B
by SAI_Peregrinus on Monday September 18, 2023 @11:04PM
(#37565179)
CloudFlare reported 10Gib/s when they first switched it on. The 10Mb/s link was deliberately limited.
The last public analysis was done in 2010 by RIPE and APNIC. At the time, 1.1.1.0/24 was 100 to 200Mb/s of traffic, most of it being audio traffic. In March, when Cloudflare announced 1.0.0.0/24 and 1.1.1.0/24, ~10Gbps of unsolicited background traffic appeared on our interfaces.
https://blog.cloudflare.com/fixing-reachability-to-1-1-1-1-g...
Make sense now why Cloudflare would be one of the only companies that could handle it!