There is general agreement that the flood wiped out almost all of mankind, but exegetes have for some time been debating whether the flood was globally universal or anthropologically universal (i.e. just flooding all the areas that man lived at the time). Lack of geological evidence for a global flood may argue strongly against the first hypothesis but does not affect the second.
But back to the article...why would the earth’s rotation slowing cause a release of heat? I don’t get that claim.
The "flood" occurred during historical times, ca. 4,350 years ago according to biblical scholars, so you need to be looking not to geology but to archaeology and sedimentology for evidence. Sediments of that age have been quite well studied in most parts of the globe, and no evidence of a worldwide flood has been found.
But back to the article...why would the earth’s rotation slowing cause a release of heat? I don’t get that claim.
If the rotational energy was changed by slowing, there would be a release of heat. But the article is wrong in equating a leap second with the earth's rotation slowing by a second. They are two different, and unrelated, things. A leap second is equivalent to a leap year, in which an extra day is added. That extra day does not mean the speed of the earth's rotation has changed in any way; it serves just to adjust our calendar to the true length of the year.