The reference for the Robertson paper is: Brown, R. M., & Robertson, E. M. (2007). Off-line processing: reciprocal interactions between declarative and procedural memories. Journal of Neuroscience, 27(39), 10468-10475.
I researched this topic a little as it was related to work I was doing in my PhD 2-3 years ago. I'm mostly going from memory here. I think Kaufman is largely incorrect for skill learning in real-life. Walker et al. (2002) found sleep improved motor skill compared to the same amount of time spent awake, but a similar improvement was found after nightime sleep whether participants learned at 10 am or 10pm.
Walker study: https://www.sciencedirect.com/science/article/pii/S0896627302007468
Possibly, Kauffman based his claim on Robertson's studies which found interference between a motor skill task and a verbal task is minimized with sleep but Robertson doesn't directly test the claim as I think he only tests the learn in the morning group after 12 hours but not 24 hours and I tend to be skeptical even of Robertson's basic finding as when I was doing research on the topic independent researchers hadn't replicated it http://www.jneurosci.org/content/27/39/10468.short.
I also found these articles that might be of interest:
Overall, spacing your practice will be a much more significant variable than the time of day you practice.