If nodes in a network are fully independent and not cooperating, then you don't have a network, you have a bunch of disconnected nodes.

Is cooperating the same as colluding?

In the sense the parent used it, yes.

If a majority of banks and businesses agree to block transactions of a known terrorist, is it collusion or cooperation? I don't see a meaningful distinction at that point, other than the negative connotation of "collusion" poisoning the well. Which is why I referred to "cooperation" instead.

The later has a negative connotation of illegal (or at least secret) cooperation. So it's a form of cooperation.

Which is exactly the fundamental flaw in any argument that "decentralization" will somehow solve problems.

Humans are social creatures and collusion is the natural order. Humans will cooperate unless doing so is clearly to their detriment.

Decentralization doesn't mean isolation and that nobody is cooperating. It's a centrality measure. More decentralized networks have less nodes that hold outsized importance in the network.

that’s not a solid assumption.

if they can pursue their own interests freely and if those interests they pursue align with the majority or the totality it does not negate their decentralized nature.