I think it might have been a glitch... There are so many other tasks that need to be done before you can get the GC, that it is probably unusual for the garden puzzle not to already be solved by the time the GC comes along.

I'm in the process of a game where I held off clearing the blockage until the rock was carved and I've earned all of the tech points needed. I still don't have the GC, but none of my villagers have been distracted by laundary in the mean time!