What actually is the percentage tolerance if resistor is 0 ohm?
In practice, it means the 0 R resistor belongs to a family of resistors from a manufacturer that all have that tolerance.
For example, a manufacturer will make a family of 1 % resistors. All the parts in this family have a tolerance of 1 %. The family includes a 0 R resistor so this is also specified as 1 %.
Mathematically, its nonsense. But it allows the 0 R resistor to be clearly recognised as a member of that family, rather than be set apart by slightly different specs. The manufacturers appear to prefer to have that consistency, reasonably enough.