I'm not trying to measure bias, the statement that I am challenging is that fox news has been moved to the left by the addition of other sources which are to the right of Fox news. This is only the case if the definition of unbiased is "average".
My reasoning is as follows:
Partisan bias is a type of bias. If there exists a way of presenting information without any bias, then it presents information without partisan bias.
if such a method exists, then we do not need to measure news sources relative to each other in order to talk about what qualifies as non-partisan. This means that partisanship is a static quality of a piece of journalism, i.e. any method you propose of measuring partisanship should always give you the same answer for the same piece of journalism.
If you interpreted fox as having a far right bias in 2004, and don't notice any particular change in their reporting, then you should interpret them as having a far right bias now.