The issue there is the cause and timescale. Waste heat from energy use isn't what is causing our current climate change, it's the effect of greenhouse gas emissions on the energy absorbed from sunlight. They're related in that the greenhouse gas emissions are generally waste products too, but they're different physical problems that seem equivalent because they have same ultimate consequence, so they shouldn't be taken as having identical solutions. Not that degrowth wouldn't be a way to solve our current issues too, but it's not the only way to, and the point where it becomes such because we reach the physical limits of the planet is a long way off. In the meantime, it has a lot of downsides to consider vs the various other ways to deal with the current problem.