He says that the commentary interprets it as: "Insight concentration, which occurs when one has abandoned the sign of permanence, etc."
He says that it is not defined further in the Nikyas but that it's placement after the formless attainments suggests it is a samadhi qualitatively different from those attained in samatha meditation. In SN41:7 there is a "signless liberation of the mind", animitta cetovimutti. In SN43:4 the signless concentration (animitta samadhi is called the path leading to the unconditioned:
... "And what, Bhikkhus, is the path leading to the unconditioned? The emptiness concentration, the signless concentration, the undirected concentration..."
These passages connected with liberation do suggest that it has to do with insight.