I knew someone would try to say that. If you take that line, then everything is cache invalidation. It's amazing the lengths people will go to to justify wise-sounding-but-wrong sayings like the above.
It's not meant to be a literal, pedantically correct statement, it's meant to be an insight, and enough people think that it is that this quote gets repeated. By all means print out your own page in 4-point text of what is and is not a hard problem and stick it on your own wall.
Consensus is most categorically not cache invalidation. Consensus is about creating new information, namely a decision about which a group of machines/processes agree.
It's not insightful if it leads people to the wrong conclusions. It would be more insightful to impress on people that CS is a large and diverse field with more hard problems than can be reduced to a single pithy saying.
Cmon, P?=NP is cache invalidation? Be serious.