> I'm sure it would notice the heartrate was up, but how is it supposed to translate that into energy consumption?

Well, you need more oxygen to burn more nutrients. This means faster heart rate.

The body normally doesn't want to waste energy, so the heart rate at any given moment is just enough to cover the requirements.

Precisely calibrating that is another story. You can go into one of the places that directly measure the oxygen use and get the baseline data (e.g. https://www.dexafit.com/services/vo2-max-testing )