Given how much trans-continental/trans-oceanic network cable the major cloud providers own, they almost certainly have special trans-cloud network traffic infrastructure. Especially since so much of "The Cloud" is within a few 10s of square miles in a field in Virginia. I can easily see how one provider could majorly disrupt another provider by accidentally breaking inbound traffic on one of those links.
The bigger issue is that there's a lot of customers where they have split cloud deployments, which means the customers hurt even if they are stable within the clouds themselves.
If you are deployed in such a way that both GCP and AWS need to be up you're doing it backwards. Multi-cloud strategy is supposed to result in the intersection of cloud failures, not the union of them.
Hope everything is resolved now for good.