This seems like an interesting claim. One way to validate it might be to run this analysis on the same topic on different news sources and see if the relative biases of the sources matches our intuitions.

For example, I would expect to find that Fox News uses more violent descriptions of BLM protestors, and that the Washington Post uses fewer; I would expect the converse when describing the Jan 6 US Capitol riots.

This kind of comparison doesn't say whether the BLM protests are more or less violent than the Capitol riots, but it would say whether such an analysis hews to our intuition about relative bias between news sources.

The question is rather, was significant bias found.