In the Swarmkeeper ranger's 3rd-level feature Gathered Swarm, one of the possible effects after you hit a creature with an attack on your turn is:
The attack’s target must succeed on a Strength saving throw against your spell save DC or be moved by the swarm up to 15 feet horizontally in a direction of your choice.
Now, a Moon Sickle gives you:
a bonus to [...] the spell saving throw DCs of your [...] ranger spells.
Does the Moon Sickle also increase the DC for the Gathered Swarm feature?
As far as I understand, the Moon Sickle either (a) increases the ranger spell save DC (and thus applies to Gathered Swarm, too) – or (b) it only increases the DC for every ranger spell (and thus does not apply to Gathered Swarm).
However, if I equip the Moon Sickle on D&D Beyond, my ranger spell save DC increases, but the DC of Gathered Swarm does not.