Short selling could be argued as the literal opposite to investing because you're borrowing stock, immediately selling and hoping it goes down in value so you can buy it back cheaper. You're not actually investing in anything.

Maybe you're getting confused with normal trades where you hold onto your stock and hope it goes up in value (like with any other investment)?

I mean, I think they were only arguing long- and short- selling are gambling, not any form of investment. I would also say most daytrading is at least gambling-adjacent. We get a little more fuzzy once we talk about investing significant amounts into a business (e.g. "Angel Investors") where you can meaningfully be said to be influencing the outcome you're betting on in a significant way, and we can probably say that investing in vehicles like index funds are a pretty safe bet as long as the economy writ large doesn't crash, but like many terms in the english language a whole ambiguous grey area exists where the connotational choice you make in describing it is pretty subjective