Not true, not true at all.
First of all, there were never any signals sent out about the attack. Not a single one, every single order from January 1941 until the attack happened was entirely hand carried. It was never discussed on the phone, via telegraph, and certainly never by radio. The very first radio message ever sent about the attack from when the operations was conceived in January 1941 was on 7 December when the lead pilot radioed back "Tiger" three times signifying they had complete surprise. That was the first form of electronic communication sent regarding the attack.
Plus, the Japanese military in their communications not only used the codes, but a secondary sequence of code words. Even if we had somehow managed to intercept plans of an attack, they never would have said "Pearl Harbor". That is why the US had to go to such an elaborate ruse in 1942 to learn that "AF was out of water". Of course, that would be of no help as the Japanese were using IJN code JN-25, which we did not break until 1942. The only code at that time we were regularly breaking was Purple, used by the Japanese Diplomatic Service.
This was common in all militaries, not only codes but also code words. That is why in both World Wars the "Codetalkers" were so valuable. Both the Germans in WWI and the Japanese in WWII did manage to capture several of the Indians that spoke the languages used. But they were worthless, as in addition to speaking in Choctaw, Cherokee, Cheyenne, Navaho, and other languages they were also speaking in code. So even with a native speaker, the messages made absolutely no sense. Like requesting an "Eagle put a potato on a turtle" and then a series of letters. That was a request to have an SDB bomb a tank, and the letters were the coordinates. But to somebody that spoke the language but did not know the spoken code that was completely nonsensical.