and for completeness, the actual command I was originally having issues with, works as well now:
pd-san2::*> statistics show-periodic -counter total-recv|total-sent|data-recv|data-sent|cluster-recv|cluster-sent|cluster-busy
pd-san2: cluster.cluster: 10/22/2021 17:27:55
    total    total     data     data cluster  cluster  cluster
     recv     sent     recv     sent    busy     recv     sent
 -------- -------- -------- -------- ------- -------- --------
   75.9KB   24.0KB   50.1KB   1.37KB      0%   22.6KB   22.6KB
    282KB   28.6KB    258KB   7.21KB      0%   21.4KB   21.4KB
    479KB    146KB    339KB   8.82KB      0%    137KB    137KB
pd-san2: cluster.cluster: 10/22/2021 17:28:02
    total    total     data     data cluster  cluster  cluster
     recv     sent     recv     sent    busy     recv     sent
 -------- -------- -------- -------- ------- -------- --------
Minimums:
   75.9KB   24.0KB   50.1KB   1.37KB      0%   21.4KB   21.4KB
Averages for 3 samples:
    279KB   66.3KB    216KB   5.80KB      0%   60.5KB   60.5KB
Maximums:
    479KB    146KB    339KB   8.82KB      0%    137KB    137KB
I dunno.  at least it works now though!