Rats! I have raised this issue to get this feature added in the next release in q2-longitudinal.
Yes — the next release is due in April so I'm afraid it will be just a bit of a wait.
In the mean time, there is an easy option: in the vizualization, there is a link to download the paired results as a TSV. So set group-column to a column that does contain 2+ different values to run this command, download the TSV, and you can then import that file in R/your favorite stats software (maybe even excel can do this) to run a Mann-Whitney U test across ALL samples.
Of course that command could still be useful if, e.g., you did have a relevant group category (e.g., you wanted to stratify by sample type or patient demographic, then see if there are paired differences by collection method), but what we are currently lacking is a way to test this across ALL samples (and the issue, to be more specific, is that the Kruskal-Wallis test should really just run if more than one group exists in the dataset).
I hope that helps!