When you mix the grounds etc with water and then let sit over night, is that water hot or cold?

Agree with previous reply, use cold or room temp water

The water is cold which is why the term "cold-brewed" is used in the author's headnote.

If you watch the video at the 0:26 mark the cook pours the water from a glass carafe, clearly *NOT* hot water.

Best of luck.