You failed to provide a link to the device you have so my remarks are based on the devices I found searching for the term.
Before I buy this sensor for my Arduino, can someone confirm that this sensor and the #include "EmonLib.h" is capable of reading and distingues current in both direction ?
Yes, the split core clamp is capable of sensing positive and negative current.
As you also failed to provide a link to the library and a wiring diagram of the planned setup I cannot answer the second question as that depends on the circuitry and the board used.
If it can, very 'high tech', as this sensor is no more than a transfo .. ( or am I wrong ? )
What's high tech about that? A transformator induces a scaled current on the output side. You just have to measure that scaled (down in this case) current but as I wrote this needs additional circuitry.