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.