Difference between "port binding" & "injecting a webserve into the environment"?

I was reading the Twelve Factor App manifesto and point number 7 mentions:

The twelve-factor app is completely self-contained and does not rely on runtime injection of a webserver into the execution environment to create a web-facing service. The web app exports HTTP as a service by binding to a port, and listening to requests coming in on that port.

I understand that for example ExpressJS does precisely this and exports HTTP as a service by binding to a port, but what would be an example of "injecting a webserver" and what do they mean by this?