But did they say it was a workout? Or just that they've done their daily steps

Are they not just looking to move more? Or are they treating it like a workout? I work from home and steps are helpful to know if I moved around enough.

If you set your step quota to 10 steps, you can both hit your quota and have gained nothing from it. This is a problem with users setting quotas too low, not a problem of the device giving bad information.

It is to some minor degree as it is impossible to know exactly burn rate. Power meters FTW!

Yeah, if you've ever seen the setup they use to actually measure your energy consumption you know some gadget on your wrist can not be accurate.

Yup, best you can hope for is consistent. That can be useful in a 'am I good, should I try more, less' way.

I mean calories themselves are what happens if you burn the food in a bomb calorimeter, which bears little resemblance to what happens biochemically in your body anyway.

Even 80-85% sounds like too much.

Compare the legs of a slim person with the legs of a severely overweight person. It's like adding shake weights onto the legs that do the pedalling.

And overweight people have different weight distributions, some have the weight on the legs some in the belly. Depending on where the weight is, that's a massive difference.

Watts input and watts output is massively different with every machine (compare e.g. an electric engine with 90+% efficiency to a combustion engine with maybe 20-30% efficiency), and humans are no different.

My Apple Watch is within 2% of the reading on the exercise bike and is noted as the most accurate in the study. I'd wager your exercise bike reading is pretty close to your actual calorie burn.