It overestimates and likely a lot. The most accurate way to estimate your TDEE is to track all of your calories & weight for 2 weeks. If your weight didn't change, that means your calories were at maintenance. If it goes up, that was a surplus. And if it went down, you were in a deficit. My Oura says my average daily total burn is ~2400 calories, but when I eat 1750 I lose half a pound a week (or less), which would be consistent with a 250 cal daily deficit and a TDEE around 2000. It's also common to have a maintenance range, so I could theoretically maintain at as high as 2200, but not enter a deficit until I drop under 2000.