The paper has a little more details:

  "following our communication with the manufacturer, a software update was issued that removed the strict periodicity of the current peaks in one battery block of the investigated UPS system"

Given the issue is so widespread (international, affecting pretty much every grid), it's likely that multiple manufacturers use similar approaches.

You see the same thing with e.g. cron jobs, where everyone schedules on xx:00 or xx:30. No communication needed, it's just natural. More senior people recognize this and add randomness to their start times or skew a bit.