Cardiac hypertrophy is not necessarily a bad thing, it can be the result of positive adaptation, such as exercising.

Eccentric hypertrophy (athlete's heart) is the positive adaptation resulting from training the heart. The heart has a lower resting rate and is more efficient at pumping blood. It returns to normal size if training stops.

You'll never reach a state of hypertrophic cardiomyopathy (the bad kind of hypertrophy) with exercise. Its cause is usually genetic.

Not true. You can't really train your muscles to use less oxygen for the same energy output (what "oxygen-efficiency" would imply). You rather increase their capacity to take up oxygen from the blood and burn it. They will use more oxygen to output more energy.

That additional oxygen needs to come from somewhere. Endurance training at the same time trains the heart to deliver more oxygen to the periphery; the primary mechanism is increased cardiac stroke volume.

This is terribly uniformed. Do not listen to this.

Cardiac hypertrophy isn't a "bad thing". This is completely contextual. What you don't want, for example, is pathological hypertrophy from things like hypertension, or exclusive left ventricular hypertrophy without associated increase in chamber size.

The heart is very complex. You 100% should exercise it.

Wikipedia says Athlete's heart is benign: https://en.wikipedia.org/wiki/Athletic_heart_syndrome

This is why I hate health science. Informed people can have the same information and come to opposite conclusions. The entire field is made up of contradictory explanations and principles, to the extent that it’s unknowable what’s true or not.

That seems… misguided.

Sources?

Your heart is also a muscle.