It is a pretty big storm and incredibly powerful. Hurricanes can cool the water that they pass over, but not to the extent that it would impact the El Nino. The storm pales in size comparison to the area of the ocean impacted by El Nino. Also, there are mechanisms in place that generate the El Nino, some known and some not, but the bottom line is, even cooling an area with an El Nino going on would be temporary, with the warmer water being regenerated. I guess the best comparison I could give as to the temporary cooling effect would be dropping a snowflake into a shallow dish of water. There would be measurable cooling, If you had the right instruments to measure at that scale, but overall, would be very small.
Post tropical systems do not pull down cold air. Sometimes they can join forces with a mid-latitude storm, such was the case with Sandy out east a few years ago, but they only add moisture and energy to such a system, not pull down cold air.
-John