Consistently factual is exactly that. Both of those words mean actual things. And they go on to say that they can’t fail fact checks. And prompt corrections likely means that as a story develops, that if there were incorrect things reported, they are corrected as soon as the new information is available.
As for who defines extreme bias, it’s literally them. That is what they are saying they are doing. And they spell out what their left vs right criteria are. And how they judge it. Of course this is subjective. There isn’t really a way to judge the political spectrum without subjectivity. They do include examples in their reports about what biased language, sources, or reporting they found. Which allows you to easily judge whether you agree with it.
As for VOA, they say in the ownership portion that it is funded by the US government and that some view it as a propaganda source. They also discuss the history and purpose of it being founded. And then continue on with the factual accuracy and language analysis. You may not agree with it, but it is following their own methodology, and fully explained in the report.
Again, there isn’t anything saying you have to agree with them. It is a subjective rating. I’m not sure how much more transparent they can be though. They have spelled out how they grade, and each report provides explanations and examples that allow you to make your own judgments. Or a starting point for your own research.
If you can define a completely objective methodology to judge political bias on whatever spectrum you choose, then please do. It’s inherently subjective. And there isn’t really a way around that.