Drupal - What does a value like node:{node_type} mean?

That definition isn't about what is passed to NodeAddAccessCheck, that is solely defined by the slug definition in the path and the type hinted argument.

Instead, it is configuration for its implementation. Which it doesn't actually use. So yes, it is pointless. See \Drupal\Core\Entity\EntityCreateAccessCheck for an access check that actually uses that definition to dynamically get the value it wants from the route match parameters.

I suppose this initially used that and was switch to a different implementation, without removing the now unnecessary configuration.