Help to verify this power supply's isolation property from its data sheet
Is its output floating with respect to earth ground on the AC side?


Yes. Absolutely. These 2 specifications show that the DC output is isolated from the AC power input by these values.


  Isolation voltage (60 sec.) – Input/Output 3‘000 VAC Isolation
  resistance – Input/Output 100 MOhm (at 500 VDC)


What they mean is that Earth ground is NOT connected to the DC outputs, therefore they are floating in the truest sense. There is no "High-impedance" connection to ground either. Many supplies have a high-impedance ground to prevent static build-up or to neutralize what few micro-amps might actually leak into the DC output side.

It is up to the design engineers and their company guidelines as to what type of Earth ground is built in, if any. This manufacture makes it clear the isolation is as good as they can make it.

This is the link to the .pdf of this line of power supplies.