Somewhat. Allsides has a whole system[1] as to how they assign bias which is fairly robust, in my opinion. You're not betting on what the moderator at allsides think a source bias is, you're betting on what all of the patrons at allsides think the source bias is which is a little better.

But you are right in that no one can "objectively" measure partisanship since there is no such thing.

[1] https://www.allsides.com/media-bias/media-bias-rating-method...