The proof of a proto Stonehenge is surely not disputed, there are clearly reused bluestones of a pillar and lintel monument, at Stonehenge itself, so there was an earlier bluestone monument somewhere that predated the sarsen monument. This is not speculation, unlike the possibility of glacial transport, which has the mechanism to do half a job, but not the means to carve and assemble lintels onto pillars.
Human intervention is a certainty, glacial intervention is not.