By looking at the bitcoin and ethereum mining pool hashrate distribution I feel that only 5-10 relevant full nodes exist. So it's more like a federated system among those.
Am I seeing something wrong here?
Yes Bitcoin and Ethereum are still decentralized. What happened is it got to risky to mine solo so miners banded together to have a better chance of getting blocks. I suppose you could say that Bitcoin or Ethereum could be federated but the fact that these pools are competing against each other destroys that idea.
(with reference to a number of states or organizations) form or be formed into a single centralized unit, within which each state or organization keeps some internal autonomy.* According to the definition of federated I would rather say that each mining pool is a federation of miners in itself. But with all these different federations involved it is still decentralized system because not one of these federations control more than 50% of the hashrate.
*Definition from Google Dictionary