Decentralization is not measured according to how many nodes there are, but in how easy and cost efficient it is for a person to run their own. Given that it is pretty easy to just download Bitcoin Core and let it sync (pruned if you don't have enough disk space), it is pretty decentralized.
And yes, miners was a synonym with nodes back in 2010.
I wonder how they count the number of nodes.
You can have a great estimation by recursively sending getaddr messages across the network.