Steem isn't being run on one server or by one organization, which would be centralisation, but on many, many different servers all around the world.
However, there is a bottleneck/weak point in Steem, which caused yesterdays problem, but all other blockchains have this in common: the code.
And regarding 20 nodes - you cant have extreme decentralisation and extreme scalability. At one point you have to decide and Steem is decentralised enough.
More on this point, if one of the 20 nodes trys to cheat or isn't playing by the rules they will get voted out immediately and lose a lucrative job so the takeaway is... consensus is still achieved in a decentralized manner.
What if an individual or individuals on behalf of a company becomes the majority witness, we saw that with EOS