Why does cold brew taste smooth when the same beans brewed hot taste sharp?

Answer: Cold water pulling fewer of the sharp compounds from the grounds

Cold water pulling fewer of the sharp compounds from the grounds ✓ — Right. Temperature is the difference: cold water is a weaker solvent, so it lifts fewer of the acids and bitter compounds that hot water strips out quickly. That is why a cold brew can be brewed strong and still land smooth. Its strength comes from how much coffee met how much water; the missing bite comes from the cold.

The long steep breaking those sharp compounds down over time — Not quite. Time does matter to extraction — a longer steep pulls more from the grounds, which is one reason cold brew can carry plenty of caffeine. But extra steeping keeps extracting rather than stripping the sharp compounds away, so the sour and bitter notes come along with everything else. The smoothness tracks the cold water, not the clock.

Diluting the concentrate washing the sharpness out before serving — Not quite. Dilution is a real step: cold brew is usually made as a concentrate and cut with water before serving. But adding water turns down every flavor at once instead of hunting the sharp ones, and diluted hot brew would still taste like hot brew. The bite was never pulled from the grounds in the first place.