How does this current sensor work (with only half magnetic core)?
"Closing the magnetic loop" improves the sensitivity of the meter, but not necessarily its accuracy. 

The magnetic field strength around a wire is proportional to the current flowing in it. However in open air this field is too weak to measure with a conventional meter. A loop of high permeability magnetic material such as silicon steel attracts the field into itself and makes the signal as much as 10,000 times stronger. This can then be transformed to higher voltage for measurement with a meter. But the magnetic core also suffers from hysteresis, eddy currents and saturation, all of which can reduce measurement accuracy.        

Alternatively the field could be measured directly using a Hall effect sensor. With good amplifiers and temperature compensation a Hall effect sensor can accurately measure far weaker magnetic fields than a moving coil meter. Without using a core the magnetic field will vary more depending on orientation and distance from the wire. But if the sensor can be accurately positioned then there is no reason why it can't be just as accurate as a clamp style meter. 

Devices such as the MLX91208 have a small amount of magnetic material embedded in them to concentrate the field, both to increase sensitivity and reduce the need for accurate positioning. Since they don't completely 'close the magnetic loop' the gain is less than a clamp-on core, but the large air gap may actually improve accuracy. 

The 'little plated pins' in the Wi-Beee may have embedded Hall sensors with magnetic material around them them that performs a similar function to the concentrators in the MLX91208. The Wi-Beee is precisely positioned over the terminal block in close proximity to the wires, so it should be able to get good sensitivity and accuracy.