A backtest shows how a strategy would have done in the past. Survivorship bias creeps in when it's run on today's companies: the ones that went bust, were bought cheaply or shrank out of the market aren't on the list any more, so the past looks better than it was. Indices change the same way through index reconstitution, dropping companies that have shrunk and adding ones that have grown.
So a chart of "the stocks in the FTSE 100" going back twenty years, built from today's members, is a chart of the winners. Fund league tables have the same problem, because closed and merged funds drop out of them. When you see a backtest or a long track record, ask what happened to the ones that aren't shown. The app doesn't run backtests, and its screener searches today's listed companies, which is exactly the list that has survived.
The failures left the index, and they left the chart.
Educational content only. It is not personal financial advice.