That assumes a “normal person” will not compensate for the effort with a snack.
A “reward” pint of ice cream will re-add more calories than the hour of cycling. A “standard serving” will nearly match your half hour.
And that’s an hour at 20mph, which really isn’t in any way the norm.
You're also assuming the same normal person will not compensate for their healthy lunch with a snack.
Measuring stationary bike workouts in terms of mph is kind of meaningless. What actually matters is the measured power output based on the resistance setting.
It took me about three months to get up to that speed, cycling every other day. I started out doing 10 miles, at 10mph.
Three months isn't all that long for an exercise program.
There are certainly plenty of trained cyclists who can maintain that power output, but I've never seen anyone reach that level after only 3 months. If you really did that then you're an extreme genetic outlier with an unusually fast training response.
I did a lot of cycling in my 20s and while I'm at least a decade out of shape I'm sure I'm relying a lot on past training.
Absolute heart rate doesn't tell us much about calorie consumption because there's a huge variation between individuals. If we know the athlete's size, sex, and threshold heart rate then we can start to make some kind of estimate. But to get an accurate number it really takes a properly calibrated power meter (or more complex lab tests involving exhaled gas measurement).
What is the actual power output in watts? Has it been properly calibrated? If not then you're just fooling yourself.
These statements are rather extreme and seem impossible to support. Energy output per mile with real world cycling has enormous variance as well. Why so much concern for precision regarding one and not the other?
I don't think there's much point in continuing here.
You can get an accurate calculation of real world cycling calories with a power meter. Everyone's cycling efficiency in terms of power output per calorie burned falls within a fairly narrow range so converting from time and watts to calories is simple. A good power meter accurate to within a couple percent can be added to any bike for about $700.
Smart trainers like a Tacx Neo or Wahoo Kickr also have accurate power meters built in. Those are substantially different from typical stationary bikes.
I understand the translation between power and mph is somewhat approximated, but I think you could be more generous. It's not unreasonable to make this kind of approximation and the numbers aren't meaningless.
I cycle on and off for ~8 years (35yo) and going at 20mph in the real world for an hour or more became possible only after a few years for me.
As a point of comparison, an average size man would have to run at a speed of at least 11 mph (18 km/h) in order to burn that many calories in 30 minutes. Not impossible, but well beyond what most of us can manage.
It is also something at most a tiny minority of the obese population can do.
Fats are the secret to obesity, hands down. I have a remarkably small appetite and I can easily down 1000 calories of ice cream in a single sitting (a single pint of Ben and Jerry's. You can row like crazy for 30 minutes, but if you have two extra candy bars, it's meaningless.
Yes, our bodies are built to take in more calories than we can burn (thankfully!), but if you keep your eating habits constant the exercise will make a huge difference.
Pretty sure most people shouldn't be eating 3000 calories/day.
A stationary bike at a steady 20mph pace is about 500 calories in 30 minutes. That's really significant. An hour will erase about a third of a normal person's diet.
When I used to do heavy training (long distance running, weight training) I would eat close to 8k calories a day and I was in fantastic shape. Eating more was necessary to survive, given how much energy I was expending.