Unglazed clay does move water from inside the pot to the outside. Obviously, glazed and plastic cannot. But I don't think this is a major factor in preventing over-watering. Roots drown faster than clay evaporation can protect against. Clay transmits water most readily when the outside surface in in contact with a medium capable of actively transporting water away from it, such as with a clay olla buried in soil or a Forsyth pot in rooting medium.
There is also some cooling effect from unglazed clay as water evaporated from the surface. Again, not a bit factor.
Some people practice and advocate removing the bottoms of pots, which greatly improves drainage by emulating natural ground. This is primarily a way to garden where hard, compacted ground is difficult to rehab. Roots do grow into the ground below the pot. But it opens possibilities like using clay chimney flue, sewer pipe, iron pipe, etc.
When a bottomless pot is set so that the open bottom is in the air and some sort of mesh contains the soil, roots will generally air prune. In any events, the open bottom provides the best possible aeration with a container. And aeration is really what we talk about when we talk about over-watering.
