It may be my eyes are going but that's nothing like I've ever seen.
A (fatigue) crack should have a starting point, or initiation point. Usually there's a clear anomaly or at least a stress concentration factor: a weld flaw, an inclusion, a hole, a machined surface, a gouge or other damage to the surface, an abrupt change of wall thickness etc.
This little crack seems to have two end points and no beginning, doesn't make sense. So what's on the backside of this surface in the picture and can we see that please?
I would not retire the swingarm on the evidence I see in this picture alone, but would persist until I understand what it is.
Or does it start at the hole and is the light so that we can't see it?
Just my 0.02$