The OP to that Reddit thread said there is overlap, so I believe him over someone speculating. The V2 tiles have a rubber gasket/lip at the top edge of the tiles (downward-facing), but it seems to me that V3 may not have that. But there's definitely overlap between the rows, and they also use a 'sidelap' that fills-in the side-to-side gap between tiles, and this also extends to the next row of tiles underneath.
So any water running down the roof would definitely keep going without getting under the tiles. If there's a strong enough wind to push water up a tile (no idea how much wind that would take, may be an unreasonable amount) then at least in V2 the lip would stop it, if V3 doesn't have the lip then I don't have a good answer there, other than that the underlayment itself is also a water barrier.
I believe you are correct. Roof tiles, as in cement roof tiles, do not have any gaskets. The depend on overlap to keep the sides and nailed upper portions dry. When the Tesla guy was here doing the site walk through he showed us the solar tiles, and there is as much overlap with them as there are with cement tiles.