I don't know why "techtarget" would be a credible source on Physics questions, but the SI convention, which is, according to Wikipedia, the "only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce", poses that "kelvin is never referred to nor written as a degree."

But I also made the mistake to write it as "Kelvin" instead of "kelvin".