in theory the intake valve has to be closed

Actually, the stoppage can be from the closed valve, or simply the difference in pressure between the inertia of the intake flow vs. the lower static pressure of the charge in the cylinder ATDC.
At low speeds, there isn't enough inertia to get flow to enter after the piston changes direction, and flow not only stops at the valve but reverses.
At higher speed, inertia keeps it going until IVC.

DCR is useful to estimate how knock-prone an engine will be a low to mid speeds, but (as said above) at high RPM the full stroke length is captured and compressed (not just the stroke after IVC) at the beginning of the torque range and DCR becomes irrelevant.