The Short Version
WHOOP added strength training body-stress analysis, giving lifters the same workout load data endurance athletes have had for years.
Runners and cyclists have had detailed body-stress tracking for years. Lifters got heart rate data and were told to figure out the rest themselves. WHOOP now measures how much a strength workout actually taxes the body, not just how fast your heart was beating while you did it. Endurance athletes got this analytical treatment first, and gym-goers are only catching up now.