Yikes, they're right:
SYDNEY (AFP) – An Australian physicist has uncovered an error in dictionary definitions that has likely stood uncorrected for a century.
University of Queensland academic Stephen Hughes found that entries for the word 'siphon' incorrectly said atmospheric pressure is the force that allows the device to move liquids from one place to another.
"It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm," he said.
I imagine some people had noticed but assumed that the definition supposed that the downward liquid created a theoretical vacuum (with volume=0) at the highest point in the tube.