In terms of accuracy, the authors mention it as a limitation, so probably it could be a problem. In terms of runtime, it should not matter. Generally speaking

In terms of accuracy, the authors mention it as a limitation, so probably it could be a problem. In terms of runtime, it should not matter. Generally speaking though, the overhead of optical flow is often overlooked. For video DL applications, optical flow calculation often takes more time than inference itself. For academic purposes, datasets are often already preprocessed and the optical flow runtime is not mentioned. Doing real time video analysis with optical flow is quite impractical though.