Thanks for the suggestion!
I was mulling this when I made group-column optional for pairwise-differences. I decided against it because then there is nothing being compared in the visualization, and the statistical tests are null. Making this column optional was much more urgent for pairwise-differences since that still performs a statistical test even on a single group!
As a workaround, it is also possible to download the paired distances as a TSV in that visualization, so you can just input some other metadata column as the group-column and then download and plot the total distribution.
I have raised an issue for this feature but it will probably take a while to get around to this — contributions are always welcome if you would like to contribute these changes to the code 