“Rent was already increasing before 1979!”
No it wasn’t. Look at the graph again. No net gain until 1979.
“But there’s a 6% annual gain, averaged out”
Why would you average it out? That defeats the objective of identifying a point of change. The author mentions that there’s been steady increase ever since 1956, but his data start almost 8 years before, but it’s trending the opposite direction that he wants, so he ignores it.
“If there is something stopping the housing market from building enough new homes for newcomers.”
Maybe it’s because three quarters of the land in the county is blocked for development.