I disagree. Spelling requires the intelligence to see a word and then have your brain remember it. Memorizing words for a spelling test tomorrow requires little. Remembering it a month or even a year later, that is intelligence.
It’s still memory. Now, if the kids had to use the word correctly in a sentence, that would require intelligence.
It’s mostly short term memory. These kids study (that is memorize) lists of words for the competitions. It’s primarily about memorization, discipline, and endurance
I don’t have the discipline or endurance, and I think most people don’t.