Yes, there are economic nodes but it’s a confusing term because it doesn’t necessarily correspond with people’s personal full nodes.

Bitcoin is a way for people and organizations of people to transact with each other. If a subset of this community disagrees with another subset on what the rules of bitcoin should be, they may not all agree on what constitutes a valid transaction.

When one subset declares adherence to a certain rule, their power to enact and enforce that rule depends on the might of their economic nodes. This has nothing to do with numbers. 51% is no guarantee of anything, whether it’s people, coins, or computers running Bitcoin Core.

Instead it’s the power of the threat to divide the network economically. How badly is the subset not enacting the rule going to feel about not being able to transact with the subset enacting the rule, and vice versa?

The magic of the UASF in 2017 was that enough ordinary users favored decentralization and scaling via Lightning that the miners were scared they’d be wasting a lot of resources mining blocks that a significant enough subset of the network would not care about. Thus they were compelled, economically, to signal for SegWit activation.

Full node has a cost. People running them have likely an incentive to maintain a copy of blockchain.