The link with Arthur's Stone is probably based on the connection between nearby Llanrhidian and Illtud - to whom the local church is dual-dedicated with St.Rhidian (the church is also next to a small cave). The link with Oystermouth would presumably be explained as being because it is the main town on the Gower, and therefore when the author says Loyngarth, what is meant is the Gower generally, by geographical synecdoche. With this in mind, one should note that the current name of Oystermouth is Ystum Llwynarth, that is, "Llwynarth Bend [Bay?]", possibly suggesting a wider range for Llwynarth itself (See Note). It should, however, also be noted that you can throw a rock pretty much anywhere in the Gower and hit a church dedicated to Illtud: for example at both Oxwich and, more pertinently, Ilston (Illtud's Stone? in Welsh Llan Illtyd).
Strangely, the church at Llanrhidian is actually associated with two extraordinary stones of its own. The first is a large coffin-shaped stone known as the "Leper Stone", decorated with humans (possible Mary and St.John at the cross*, or the Saints Paul and Anthony*) and animals (possibly bears)(*p.173). This is now in the church porch, having been dug up nearby. Suggestions as to its age and origin range from 9th C Irish to 10th C Viking. The second is a block of masonry on the top of the church tower which is said to have been used for the site of beacon fires (*p.173).
Plainly the chief reason for tying the Arthur's Stone and the Wonder is that Arthur's Stone does
seem impossibly held in the air by the smaller stones under it. Crawling under it is a
quite oppressive and nerve-wracking experience. There are other dolmens on the Gower,
but the scale of Arthur's Stone has led to its fame, and this probably led to
its association with the Wonder. The current vista from its location only adds to its effect.
The stone has plainly been famous for some time. Carlisle*
The famous Druidical Monument, called Arthur's Stone, mentioned by Camden. This reference isn't
forthcoming in the 1607 edition of Camden or the 1610 Holland translation*,
but perhaps might be found in a later revision.
There appears to be some confusion over the name, and, unfortunately the whole thing may have been contaminated by the 19th C. Druids movement. The name Maen Cetti ["The Stone of Cetti"] has no reliable myth attached to it. It appears in the very unreliable* Y Myvyrian Archaiology (See Other Works) "Third Series" triads, translated by Iolo Morganwg as*:
Triad 88: The three Works of Might/Mighty/Stupendous works of the Island of Britain: the erection/erecting of the stone of Cetti; the work/erecting Stonehenge/of Emrys and the raising/piling of the Mount/Heap of Battles/(of Battles?) of Cyvrangon.
Now it is possible, as with Emrys, that there is some independent tradition here that Iolo has drawn together, or it may be his personal "embellishment". Lewis* quotes the following from Western Antiquary, November 1884, supposedly published about Lanyon Quoit in Cornwall (Info; Info), though possibly confused with this location:
...in it were performed various ceremonies relating to the Bardic orders, with a reference to the great event of the deluge, and the primary one of these was the sheathing of the sword as a token of peace, and insulated from all the parties and disputes of the world; being perculiarly dedicated to the Arkite genius, it was entitled Maen Cetti, the stone of Cetti or the Ark, and the raising of it was, according to the triads, one of the three mighty labours of the Isle of Britain; in it were celebrated all the mysteries of Ceridwen or Cetti, and in it her mystical cauldron was said to be warmed by the breathe of nine damsels...Here the adventurous aspirants beheld some of the mysteries of Druidism, when admitted behind the veil which on such occasions was hung over its entrance.
There may also be a distinction between "Arthur's Stone" - the capstone - and Maen Cetti - the supporting stones or hollow. Green* quotes Ashe as giving the following:
Legend has it that when Arthur was walking through Carmarthenshire on his way to Camlann, he felt a pebble in his shoe and tossed it away. It flew seven miles over Burry Inlet and landed in Gower, on top of the smaller stones of Maen Cetti.