https://www.investopedia.com/terms/b/backtesting.asp

https://en.m.wikipedia.org/wiki/Modern_portfolio_theory

With extreme synthesis :

markets are not stationary environments so you have to expect and mitigate drift. This have implications on the evaluation methodology and on the choice of time series models that can be calibrated with fewer data points.

A strategy to make money in the markets allocate capital on multiple financial instruments using multiple signals therefore the value of a signal is the predictive advantage that it provides when stacked on top of others commonly used signals. If the predictive capability of the news sentiment is easily replicated by a linear combination of cheaply available signals then it's not worth much.