How can Diamond say with any certainty that food production must precede writing? Why might people not develop writing earlier than food...

Diamond can say this based on two things.  First, he can
base the statement on the historical record.  So far as we know, no hunter-gatherer
society ever invented writing.  Second, he has a theoretical reason why this would never
happen.  He says that writing is only needed by sedentary societies that produce their
own food.

A relevant quote can be found on p. 236 in my
book, about 2 pages from the end of the chapter:

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Writing was never developed or even adopted by
hunter-gatherer societies, because they lacked ... the institutional uses of early
writing...

Why is writing
necessary?  It's first necessary for keeping records.  That is how it always arose. 
Writing arose as a way of keeping records for the governments.  These records tended to
be records of how much of various supplies a government had and how much various people
owned (so they could be taxed on it).  A hunter-gatherer society has no need for such
things.  There are no large surpluses to be stored up in such a society.  There is no
government bureaucracy to keep track of such things. Therefore, there is no need to have
writing.

It is also worth noting that Diamond says that the
food surpluses are needed to feed the people who write.  Writing was always the job of
specialized people who had the time to learn to write.  These people had to be fed from
stored surplus food.  They could not have the time to learn to write if they had to be
out hunting or gathering.  Therefore, food production was necessary in order for them to
exist.

So, food production must precede writing for these
reasons.  The food is needed to feed the specialists who do the writing and there would
be nothing to write about until food production created surpluses that needed to be
recorded.