Is this a fully functional concert performance instrument?

You can control pitch like you can with an ocarina by adjusting your breath support. It is possible that you might run into pitch issues depending on manufacturing processes. These instructions were primitive early instruments so they only have about an octave range similar to an ocarina

Is it really only 10cm tall?

That is correct. It's not big, but the sound is bigger than you would think.

Since it is made from a porus clay you don't really need to clean it but if you would like to clean it when you first get it. I would cover all the holes and fill it with warm soapy water. Give it a few shakes, uncover the holes and shake out the water and rinse with warm water and repeat till water runs clear. A side note there is an upper chamber and a lower chamber so depending on how the chamber is designed it could also be best to submerge in soapy water then rinse with water. You should not have to do this often except for the first time you get it. These are like ocarinas and that they don't really require that much cleaning maint.