The Summerland is not old. Andrew Jackson Davis, an American seer of the mid nineteenth century, described the afterlife in a long series of trance-produced works as a place of continued growth and learning, and the name Summer Land belongs to him and to the Spiritualist movement that took him up. It replaced judgement with education: the dead continue, improve, and remain contactable.
That last part is the point of the whole system. Spiritualism was built around communication with the dead, so its afterlife had to be near, benign, and populated by people recognisably themselves. A Summerland where the dead garden, study and greet relatives is exactly the afterlife a seance requires.
Modern Wicca took the name and changed the frame. Where Spiritualism was largely progressive and one-directional, Wiccan use places the Summerland between lives as a place of rest before rebirth, which is a reincarnation model that Davis and most nineteenth century Spiritualists did not hold. Gardner and his successors were assembling from what was available in mid twentieth century Britain, and Spiritualism was very much available.
The lineage matters because the Summerland is often described as an old pagan belief. It is a documented Victorian one, adopted a century later, and the documentation is not hard to find.