Are consensus (and atomic broadcast) protocols limited by single node's resources?
Your argument for "no" is flawed, for two reasons:


When we say that X reduces to Y, we mean that a solution to Y is one way to solve X.  But there might be other ways to solve X that don't rely on solving Y.
Some protocols for consensus elect a leader, but that doesn't necessarily imply that all approaches to solve consensus require electing a leader.  (For example: consider Bitcoin.  No leader, but it arguably solves a consensus kind of problem.)