This seemed implausible so I looked into it, and it's wrong as stated (at best, it needs to be made more precise to capture what you intended). First, you've mentioned Cloudflare, but the equivalent AWS product (CloudFront) does support TLS 1.3 (https://aws.amazon.com/about-aws/whats-new/2020/09/cloudfron...).
HN isn't behind CloudFront, though, so you probably mean their HTTP(s) load balancers (ALB) don't support TLS 1.3. Even that's an incomplete view of the load balancing picture, since the network load balancers (NLB) do support TLS 1.3, https://aws.amazon.com/about-aws/whats-new/2021/10/aws-netwo....
echo|bssl s_client -connect 50.112.136.166:443 -min-version tls1.3
Connecting to 50.112.136.166:443
Error while connecting: TLSV1_ALERT_PROTOCOL_VERSION
94922006718056:error:1000042e:SSL routines:OPENSSL_internal:TLSV1_ALERT_PROTOCOL_VERSION:/home/bssl/boringssl-refs-heads-master/ssl/tls_record.cc:594:SSL alert number 70