[roenxi]

> But doesn’t say why ... slow metabolism gives any evolutionary advantages

I imagine the author accidentally thought it was obvious. Having a slow metabolism is an evolutionary advantage in the same way not spending money is a financial advantage. It uses little energy and energy is usually the limit on what animals can do. If you don't spend much money, you don't need to earn much and can just exist. Being a high-earner-high-spender is probably a more efficient equilibrium, but spending nearly nothing works well enough to survive and if someone can spend less than they earn eventually they get wealthy.

If sloths don't do much, they don't need to eat much to balance out the energy expenditure. If they don't need to eat much, they can take less risks and tend to survive. Being slow is one aspect of their strategy of not needing to eat much.