Did you even read the article? Its about lava tubes in Saudi Arabia hosting hunter-gatherers, herders, and other generations of humans from the desert heat. Its not some inner earth conspiracy theory. Also why would the Smithsonian host Inner Earth conspiracy theories? From the article,

In the lava tubes, archaeologists discovered human bones, animal remains and stone tools. They also found rock art featuring cattle, sheep, goats and dogs, which aligns with ideas that prehistoric people of the region kept livestock.