Quite easy to run into hardware, OS or especially language runtime (looking at you Ruby, Python) limitations when pushing even moderately high traffic even for totally stateless applications
I certainly wasn't suggesting that you can vertically scale a service to handle unlimited traffic on a single instance.
The point is that if you have stateless services, where the resources being consumed per request are mainly CPU and network bandwidth, then "scaling independently" is not a useful thing to do. You can just scale everything instead of artificially restricting which instances can handle which kinds of requests.