False assumption.
Most cave explorers and miners used candles (a very small fire) for thousands of years. Before that, torches were used.
Small fires of particular materials produce very little smoke and use very little oxygen. The article explains they found evidence of fires in the areas they were searching.
Candles date back 3,000 years.
European cave art was created 30,000 years ago.
The fossils in the article were two million years old.
And, while making the connection from an outlier into an unexplored main passage of The Mammoth Cave, literally miles from the entrance, bare foot prints were observed in the mud.
That event occurred within the last 50 years. The bare foot print was made unknown years and years ago.