The validator website includes the message that "Note: Selecting a dataset only performs validation. Files are never uploaded."
However, when data is selected for upload the browser counters with something like this (Chrome):

Or like this (Firefox):

As noted by @robertoostenveld, this is not in the validator code-base, but comes from the browser itself. The question is whether there is any way to prevent the browser from raising this, or some other way to mitigate this.