Of course they had to take a stand for Biden, the only other choice was a lunatic. Love him or hate him, one thing Trump is, objectively, is a bullshitter (in the academic sense of the word). And scientists really just don't like bullshit.
There was an obvious third option: not endorsing anyone. There's no law that requires every publication to endorse a presidential candidate. In fact, most of them don't do that.
If you get the feeling that Nature is politically biased (and I mean this in the usual everyday person sense, because of course any politics that affects science will be met with strong views), I think that should serve as a signal to check what your biases are.
I'm sorry, but this feels like gaslighting. GP has listed numerous examples of editorials that were biased in favor of a certain political platform, including an explicit endorsement of a presidential candidate. I really don't understand how, in spite of that, you could arrive at the conclusion they aren't politically biased.
Remember that the people reading this are all the top experts in their own fields, so you can bet they'd love to write back and argue if some editor wrote something stupid.
Only if they don't mind committing a career suicide.