Earlier quoted context omitted.
That is possible if their editorial staff honestly believe two things: - The rule of law and the republic as a process are superior to anything else. - One of the candidates does not subscribe to these sufficiently. In that case, and given the political duopoly, they could still claim to be impartial towards candidates. Their only allegiance/partiality would lie with the rule of law and the republic as a process.
You’re thinking about this too hard. It’s a definitional question. If you have two candidates and you endorse one of them, you’re de facto partial with nothing else playing any role in the definition.
Endorsing on a specific issue, party, gender, or any specific quality that a large number of individuals share, is not uncommon. The specific individual is not what the endorsement must be partial to. Maybe the choice (since it's inherent to the process), but that's the distinction that was being made.