Nope, Isaacson did not write that, nor did anyone else. Isaacson quoted Steve's sister's book of fiction based loosely on Steve, based on what she thought she knew of him. The quote is: "He was a man too busy to flush toilets." So even if it is true about Steve, there's good reason. And it doesn't necessarily mean he never or even rarely flushed the toilet... or that he even failed to flush more than twice!