It doesn't mention anything about what a write unit is, except to say you can reduce write units by batching inserts (that part I guessed already.)

There's no way to think about what an actual write unit means. You could measure the costs on a sample workload, but that's far from ideal. Some transparency here would be nice.

I understand the answer is complicated, based on hairy implementation details, and subject to change. Give me the complexity and let me interpret it according to my needs.