Having a small impact is not the same a having zero impact.
Traders operate over longer timeframe even if they are holding an individual stock for minutes there is a limited pool of stocks. Keep playing the game and longer term impacts add up.
But do the fundamentals of an individual investment matter in the short term compared to the fundamentals of the market as a whole? I would suspect that the “longer term impacts add[ing] up” would just be market health. Day-trading randomly-picked stocks with random buys and sells is a Markov approximation of an index fund :)
I think that depends on how stocks are chosen when you are day trading. Limiting things to high volatility stocks for example creates a bias in your index fund approximation.
Having a small impact is not the same a having zero impact.
Traders operate over longer timeframe even if they are holding an individual stock for minutes there is a limited pool of stocks. Keep playing the game and longer term impacts add up.