Day trading is gambling. Buy-and-hold is investing. Algorithmic micro-trading is investing, despite looking like gambling. It's about whether there's an immediate dopamine release or not.
Now I don‘t consider day trading to be gambling (even though it is packaged and marketed to appeal to and sell to gambling addicts). For it to be gambling, it would need to be a game.
It is something far more sinister and far worse then gambling. I consider it exploitation, or more accurately capitalist exploitation. This also applies other aspects of the stock market, even the ones which are not marketed to exploit gambling addicts, including investment backed retirement funds [401(k) in the USA], index funds, and publicly traded companies. To me, all these things are equally bad (well actually the retirement funds may be the most sinister of the bunch) even though day trading might have the ugliest manifestation. These are all exploitation of the working class.
It's skimming.