My tyre has hollow knobs, is it normal/desirable/essential for maker to disclose?
I don't think the tread blocks are 'hollow' exactly. 

Tires are made up of layers of different formulas of rubber. I think you have worn though the outer layer and a little chip of rubber has flaked out of the center of some of the the blocks. Now you can see that the next layer down has de-laminated from the outer a little, so it looks like there is a void there.

If the blocks are hollow, the void is only right at the bottom of the block, and would only be exposed if the tire is very well worn, as yours is.