What? Your numbers are right, if you were running the game at 100FPS it would take 10ms to render a frame. Plus your 10ms of additional latency from holding the frame. 10ms + 10ms is 20ms.
If you were running the game natively at 50FPS, it would take 20ms to render a frame. That's the same number. The total input lag from rendering is identical. Add in the slowdown from your GPU rendering the in-betweens and it's even a little bit worse.
VSync may complicate this though, depending on the method, since you may already be holding a frame for some amount of time, I hadn't considered that. I personally use VRR, so it isn't much on my setup.