Would this be considered a valid reason to apply for deferral for graduate school admission?
You can ask the program what their policies on deferrals are, but I wouldn't defer for this reason.
Your goal at this point is to move to the next step. You've been admitted to a PhD program, so you completed that transition, it's time to start working towards your PhD.
It's perfectly reasonable to finish up previous work as a side project (you don't want to delay progress on your PhD, but it's worth dedicating a bit of extra time to polish up a manuscript and get it through review). Assuming your start time is fall 2021, you still have several months to push the project as far as you can. I would strongly suggest that approach rather than trying to buy another year, even if it's permitted to do so (it may not be).