It doesn't run a full OS. Merely the nodejs binary with all it's dependencies.

You can think of Docker containers as a way to package an application dependencies, ending at the (Linux) kernel API/ABI.

"Merely".

As in: "a completely different node js binary in its own environment, its own networking, its own file system overlay etc."

I cannot say if your are wrong because of ignorance or bad faith, but in case it's the former: running in a container is not the same as running in a VM, no rewording of the definition of either will make it the same or even equivalent. The difference have some impact on performance and security among other things, I recommend that you take the time to understand the difference if it's a topic of interest for you.