There are many forks of Bitcoin, like Bitcoin Cash or BSV. When core developers suggest "what is valid is what is accepted by all these nodes," they're referring to the fact that nodes ultimately determine which cryptocurrency they acknowledge as valid for transactions. This could range from Bitcoin, its various forks, to even Ethereum and beyond. It's not a part of the protocol itself but a kind of very metaphorical vote, akin to how you "vote" for Canadian dollars by accepting them as a form of payment.
> nodes have been used to influence policy
Which policy are we referring to? There is no centralized authority here. Each participant has the autonomy to decide which blockchain to support. If a Bitcoin fork appeals to you, you're free to use it. The only semblance of "politics" that emerges is the debate over semantics, the "true Bitcoin," but this isn't dictated by the protocol. It evolves in a decentralized and organic manner, devoid of any formal political process or voting.
It bears very little resemblance with typical notions of voting and politics. It's only political in the broadest sense of the term, when starting from the axiom "anything involving humans is political". Which dilutes the significance of the term 'political' - a trend that I personally find very irritating, hence my initial snarky comment.