If your initial portfolio is 100k you are not going to have meaningful "market impact" with your trades assuming you actually make them vs. paper trading.

Lack of market response is a valid point, but $100k is pretty unlikely to have much impact especially if spread out over multiple trades.

the quant trader you talked to probably sucks.

I mean if you’re going to write algos that trade the first thing you should do is check whether they were successful on historical data. This is an interesting data point.

Market impact shouldn’t be considered when you’re talking about trading S&P stocks with $100k.

Historical data is useful for validation, don't develop algos against it, test hypotheses until you've biased your data, then move on to something productive for society