Yes the vast majority of the money is American. The separatist movement is in open collaboration with American politicial and corporate actors. Some 60% of our fossil fuel industry is American-owned and there's cross-polination between it and the separatists. Of course there are other foreign actors and it's good to know their influence but the US is unmatched in scale.
Some news outlets specifically hammer the non-US actors to distract from the elephant. The Walrus isn't one of them AFAIK so their reporting is indeed intended to inform. I wouldn't take it to mean that America is not the main problem in this context. Well, American capital, not so much American workers.