the recent dos + inflation bug

That would be CVE-2018-17144

Can you someone elaborate on the economic majority?

A definition from Concensus as a service, Swanson, 2015

A consensus mechanism is the process in which a majority (or in some cases all) of network validators come to agreement on the state of a ledger. It is a set of rules and procedures that allows maintaining coherent set of facts between multiple participating nodes. In the case of Bitcoin, the “longest chain” – the chain with the most proof-of-work – is considered to be the valid ledger.

Active nodes reject transactions or blocks that don't meet the rules. If some nodes have a different set of rules, there can be effectively a fork in the blockchain.

Ultimately it is the longest version of the blockchain that prevails. This has the most proof-of-work embodied in it.

In the case of the bug mentioned, as more nodes get upgraded, they will reject any blocks that contain transactions that attempt to exploit the bug. There will be invalidated blocks.

Only active nodes play any part in enforcing concensus rules. Nodes which are not currently propagating transactions or blocks play no part in this.