Nginx is quite scalable with the right configuration and hardware, but most of the times you want to use more instances of it to scale to this kind of traffic.
Modern, Envoy based proxies are quite popular, and can scale better with better performance, mostly due to the fact it’s written in C++.
Different needs for different usecases where NGINX is more of an OOTB situation. NGINX is great. I see Caddy mentioned quite a bit, believe it’s written in Go where NGINX is written in C. Cloudflare wrote Pingora in Rust IIRC and I want to say they are releasing an open source flavor
You can scale horizontally with a lot of web servers and load balance them. You don't need to stretch load balancer clusters across AZs/regions, so I see no reason why Nginx wouldn't work.
I assume at that scale session cookies are shared without application server clustering. A distributed KV store is probably enough.
They also wanted something written in a memory-safe language so it could be extended without easily falling into memory safety issues. NGINX is written in C.