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 70Version: 2.0.7 OpenSSL 1.1.1n 15 Mar 2022
Connected to 50.112.136.166
Testing SSL server news.ycombinator.com on port 443 using SNI name news.ycombinator.com
SSL/TLS Protocols:
SSLv2 disabled
SSLv3 disabled
TLSv1.0 enabled
TLSv1.1 enabled
TLSv1.2 enabled
TLSv1.3 disabled