Is sheathing required under a hybrid concrete tile roof?
Its odd looking, but the batten support lets the tiles 'notch' into place.  You'll see the same support for clay tiles, also.

Because of the 'hanging' aspect of the tile, most locales only allow tile roofs for pitches between 4/12 and 6/12 (rise over run).  Lower pitches are possible, but then waterproof membranes are required.  In those cases, sheathing support for the membrane would be needed.

You don't mention it, but not using  sheathing is usually only done in warmer climates, where summer heat is the main problem.