You're exactly right. "Diagnostic level" of testing would tell you the difference between 120 and 121 bpm. Fitbit would read 85 bpm. That's not even close to right, which means it's completely useless for what it's marketed as.
With the same logic as Fitbit, my watch tells reads my heart rate. Right now it's saying my heart is beating 9:21am. Sure, that's not as accurate as a real medical device, but it's a number so it's in the right ballpark.
With the same logic as Fitbit, my watch tells reads my heart rate. Right now it's saying my heart is beating 9:21am. Sure, that's not as accurate as a real medical device, but it's a number so it's in the right ballpark.