The ark wasn't made of a single tree, but, presumably, much, much smaller planks fastened together somehow.
My understanding is that the
main problem with a wooden ship that size is that the you cannot prevent the planks separating when the sea is anything by glass smooth, allowing a lot of water to be taken on board.
Again, I will repeat this: it would be very easy to demonstrate that a wooden ship of these dimensions is sea-worthy -- build one! I am sure the ICR and AiG
bilk collect a lot of money from their followers. ICR fancies itself a research organization -- it would be interesting to see it do some actual research.