Had previously been used for another devious plan of the IJN Submarine force earlier in the war. Totally unopposed, they were unable to light a forest afire, in Oregon, in the late summer (when trees are usually very ummmm, flammable) of 1942. And now you think they will be able to successfully bomb one of the most densely defended locations on the planet in 1944-45? It doesn't matter if the I-400 class SS's get within launch range of the canal undetected, those planes had zero chance of accomplishing any significant damage against the canal.
Actually, the forest in question was a Pacific coast rain forest, so it was never flammable to begin with.
I think that says more about Japanese strategic planning ability than anything else.