I thought the concept of sigmoid curves modeling population was well known at that point in 1960. Perhaps human exceptionalism meant people thought it wouldn't apply to us.

We thought the curve would level out when we ran out of food, same as with rabbits.

  The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now.

It seems the authors were publishing outside their main area of expertise.

"The authors are members of the staff of the department of electrical engineering, University of Illinois, Urbana."