piezo sensors's resonant impedance
When the piezo resonates it's impedance changes dramatically and, at resonance, according to the data sheet linked, it looks like a resistance of 500 ohms (maximum). This doesn't tell you nominally what the resistance is of course. The piezo exhibits an impedance that looks somewhat like this: -



The lowest point on the graph is the point where the resistance is specified at 500 ohms (max).


  Is it the same as the piezo transducer's insulation leakage resistance
  as shown in this link?


Probably not, but you linked to a wiki page with a lot of detail and I never caught sight of what you referred to.


  And how does this resonance impedance effect the response of the
  sensors.


At resonance, the piezo exhibits an impedance that makes it easily driven electrically. In simple terms the L and C in the left hand limb of the equivalent circuit cancel out leaving just the series resistance across the terminals and this is the resistance specified in the data sheet. It's a little more complex than that because of Cp also forming a parallel (anti-resonance) impedance very close to the series resonance.