It may seem counterintuitive, but life on Earth, even with all the messy erosion it creates, keeps continents growing.
It had never occurred to me to consider that life might cause erosion. That's usually what wind, rain, and gravity are famous for, isn't it? Plant life is pretty famous, surely, for countering erosion by stopping soil getting washed away (a lack of which leading to occasionally disastrous consequences in flash floods, for example).
The sediments, like milk-dunked cookies, carry liquid water in their pores
Milk-dunked cookies don't carry liquid water in their pores. They carry milk. So the sediments are more like water-dunked cookies, moreso because they both taste yucky.
But over time, if life never evolved on Earth, not enough water would make its way to the mantle to help produce more continental crust, and whatever continents there were would then shrink.
Now, continents cover 40% of the planet. Without life, that coverage would shrink to 30%. In a more extreme case, if life never existed, the continents might only cover 10% of Earth.
That's very confusingly written. The first sentence say "if life never evolved on Earth...continents there would then shrink." But then how did those continents get so big in the first place? Surely shrinking continents is only the case when life did evolve, but then theoretically all dies off.