The Short Version
The claim is partially true but misleadingly broad. Reputable medical sources like MD Anderson and Keck Medicine of USC do recommend sunscreen indoors — but specifically when you spend prolonged time near windows, since UVA rays can penetrate glass. However, Cancer Council Australia and other authorities say indoor sunscreen is "typically" unnecessary because overall UV exposure indoors is low. The blanket phrasing "even when indoors" overstates what is actually a conditional recommendation tied to window proximity, skin conditions, and exposure duration.