This is probably true for some stutterers. Anecdotally, I'm not sure the two are exactly linked biologically. I could see other factors influencing this relationship (between stuttering and rhythm deficit) *like misfiring and glitching of the speech mechanism that causes stutterers to rely on their left brain to handle the load, which could possibly result in some deficiencies in rhythm compared to the general population*, But Kendrick Lamar, Ed Sheeran, Elvis, and Noel Gallagher from Oasis all basically use rhythm at the highest and most advanced level with their lyrics. It's also known that stutterers often sing perfectly fine. Did that study wind up finding any biological relationship?