yeah, they aren't very active, but (presumably due to federation bugs) there is more there than your instance is showing you: from my perspective the most recent post on the mander community is from one month ago and the lemmy.ml community has three posts including one that isn't from a mod.
you might be able to pull those posts into your instance by searching for their permalinks there (which you can find from the fediverse icons on each post in the web view of those communities on another instance).