You can't take every good short story and turn it into a novel, and not every novel could be meaningfully reduced to a short story. Concepts that work in one format won't work in another.
Probably true but if you have several popular short stories done in a certain style or genre then it’s a good bet that a novel in the same will be more popular. They’re good practice too, the article mentions that a lot of the respondents have only published a single book.